Lights can be split into types Low pressure sodium, High pressure sodium, Fluorescent, High pressure mercury, (All called discharge lighting) Halogen incandescent, Basic incandescent, and LED. Because High-intensity discharge lamps are not recommended for periods of less than one hour and are designed to be some distance from what they are illuminating with the exception of Metal-iodide which may be used for garden lighting of the discharge lighting in the home only fluorescent is really used. The fluorescent and be sub divided with compact self starting units and those where independent running gear is required.
Because of Part L requirements builders are forced to use versions with independent running gear so the bulbs (Electricians call them lamps as they say bulb are plants but others feel bulb referred to bulbous shape and any device replacing the original basic incandescent device should go by the same name) can't be replaces with incandescent types.
In spite of government moves to try to persuade people to stop using incandescent devices the small spot light seems to be still popular and retail outlets who have said they are going to stop stocking incandescent lamps seem to still stock the bulbs for these small spot lights.

Small spot lights
With incandescent types these can be split into extra low voltage and low voltage (Low voltage is 50 to 1000 volts AC) the extra low voltage have advantages where low voltage lamps could be dangerous i.e. around water and if and only if a inverter rather than transformer supplies them better able to cope with voltages variations.
Inverter An inverter changes the 230 volt AC to DC charges a capacitor then turns the DC to AC at high frequency then transforms it to voltage required this incorporates a switch mode method of control and can compensate for voltage variations they are often called "electronic transformer" and often dimmable with special dimming unit. 
Dichroic have a special multi-layer coating on the reflector of the lamp this means that visible light is reflected forwards whilst the heat is transmitted backwards through the rear of the lamp. The GU10 base at top of picture on left has a slight taper as it reaches edge of holder where GZ10 below has square shoulder so non-dichroic GU10's will fit either holder but a GZ10 dichroic will only fit in a GZ10 holder so only fitting able to deal with the heat coming through the back of the light can be used. With 12 volt spot lights the difference between both types is not so easy to see and it is easy to use the wrong bulb.
There is also the angle of the bulbs beam 10°, 24°, 36°, 60° and colours and size GU4 and GU5.3
There are LED and cold cathode (fluorescent) versions that well fit in the same base but the lamp is often longer and can end up sticking out of the fitting.
Quartz halogen lamps produce a little more light per watt than a standard tungsten bulb and have been advertised as energy saving but it is so small compared of a saving it can be discounted.
With the wrong distribution or angle they can produce shadows and except where a skilled lighting expert is arranging and selecting the units they are only really for show and main lighting is still required.
Because of the numbers of lamps on the same circuit there is a real danger of overload. To the right is shown the inrush chart not really important as to which bulb this refers to it is more to show the problem. For the commercial setup there are all sorts of devices to assist with this problem but in the domestic market it is normally catered for with multi switching even if all the switches are switched on together there is likely to be enough milliseconds between each one to prevent tripping. Unless otherwise recommended by the manufacturer, a small spotlight or projector shall he installed at the following minimum distance from combustible materials: (422.3.1 + 422.4.2)
(i) Rating up to 100 W                 0.5 m
(ii) Over 100 and up to 300 W     0.8 m
(iii) Over 300 and up to 500 W    1.0 m
Luminaires marked accordance with BS EN 60598-1 are suitable for mounting on a normally flammable surface. similar to F but where a
thermal insulating material may cover the luminaire. The   means only can be used on non flammable surfaces. Remember any metal item that can be touched should be less than 80°C Table 42.1 some lamps need mounting high to comply with this. Other symbols are used by manufactures for example Luminaires are suitable for direct mounting on materials that have normal or reduced flammability, in which you may not exceed 95°C (in normal operation), 130°C (in abnormal operation) and 180°C (in fault case) for less than 15 min.. Maximum temperature on the mounting surface: 130 °C. Luminaires are suitable for direct mounting on materials for which the flammability properties are not known, in which you may not exceed 95°C (in normal operation) and 115°C (in abnormal operation fault case).

Wiring methods
After the second world war many methods were used to save copper and the ceiling rose became adopted as a method of both suspending the lamp and connecting the cables. The method of taking the line feed to switch through the ceiling rose is very handy where emergency lighting is required. Klik part of the Hager group produce a ceiling rose used commercially where the lamp can be just plugged in there have also been many lighting tracks which will allow the same but in the domestic market cost as precluded these and the cheap ceiling rose is used at less than half the price of the plug in unit. The main problem is where the brown sleeveing on the blue switch wire is missing and neutral and switch wire gets mixed up. I did at one time save instructions on how to use lamp itself as a test lamp to find which wire is which but many seemed to make mistakes while trying to follow the instructions so I have deleted it. Other sites may help. With power off testing (ringing as originally a bell, battery and couple of  wires would be used) seems best method until switch wire is located.
There are of course many methods that can be used to wire a house. Junction boxes or even using the switch box as a junction box can be used. One problem is where screw terminals are used there must be access to the junction box. Often hatches are made in the floor and marked junction box below but once carpets have been laid it is unlikely anyone will find these without some form of plan. And not really accessible. Electricians don't have to follow standard wiring and one may find many different plans. All Line wires should be brown although using a sleeve is allowed therefore there is no colour code where three phase coloured wires have been used for house wiring as common with two way switches. Intermediate switches as shown left can also be used as two way one connection left unused. I note MK show Yellow now Grey as common to common wire. Red now Brown as S1 to S1 and Blue now Black as S2 to S2. And refers to method I show as "3 wire control" this is most likely followed by many electricians but it is not a standard. Some old (pre 1966) installations have no earth see this for more information.
 

Light/movement/heat detectors
Both indoors and outdoors sometimes automated light control is required. Switching lights on and off on a regular basis will reduce the life of many bulb types and the question must be asked if a 500 watt halogen lamp switched on 20 times in a night for 5 minuets a time which would average 70 watts over 12 hours would it not be better to run a 70 watt Metal-iodide all night long? And even these would in most cases be considered as being too bright. A 9 watt energy saving bulb is enough to see ones way at the side of my house once it is warm but discharge lamps are quite a bit dimmer when first switched on. This idea has also been voiced by the home office. So outdoors a standard photo switch like shown to far left or next to this with timer to switch off after set number of hours seems better options. There are also lamps with sensors built in. The PIR shown to right does take some careful setting up with masks etc. Some lamps come with combined light and movement sensors which are programmed by switching the supply off and on so many times these of course are messed up by power cuts. Moving indoors bottom left is the type of PIR used for stairs and landing. Here a 2 Way + Off switch is used allowing either PIR or switched operation. A diagram to right shows two different ways to wire. If the power is permanently on to the PIR it will sense any movement and always be switch to correct mode so flicking switch between permanent on / off / PIR controlled will get imitate correct response. But using link within PIR may cause some delay in PIR switching on. However it would need 4 cores to have independent power so in the main one would use manufactures link inside PIR allowing standard triple and earth cable to be used. Position of PIR would dictate this as if close to ceiling rose neutral supply may go direct. The problem for those following is the switch would look so similar to standard two way wiring mistakes can be made. Also any electrical item can go faulty and consideration should be main on how to isolate the PIR top left is so easy it just unplugs and a plastic bag to stop rain getting in would mean it can be isolated without a switch but in most cases an isolator is required.

Earths
Years ago it was common not to connect earths to lighting supplies and where the switches and ceiling roses etc where plastic it did not really matter. Since there was no real problem many electricians would miss out testing the lights especially since it often required dragging around steps. So although one shouldn't find any unearthed lights it is still quite common. Because 543.6.1 requires the earth to follow the same route as power cables and 543.1.1 requires earth wires not incorporated within supply cables to be at least 4mm² there is as much work add an earth as renewing cable so to correct missing earth a rewire is required. In the mean time only items marked with can be used. The sign means it is Class II equipment and does not require an earth. Although isolation transformers could be used it would be better to use an extra low voltage system if these were going to be introduced. The Electrical Safety Council do a number of instruction sheets on how items like this should be dealt with see Best Practice Guide 1.

Protective devices
Because many ceiling roses are only rated 6 amp although 559.6.1.6 allows up to 16 amp on a lighting circuit. With the requirement for RCD on bathroom lights and buried cables rules many lighting circuits will also have RCD protection as well. When a light bulb blows it often causes ionisation and a resultant short circuit the MCB's have a magnetic part which can react to this short circuit quicker then the built in fuse that should be incorporated into every light bulb. Regulation 314.1 tells us how we should split the installation up into circuits for a number of reasons including safety and as a result upstairs and downstairs lights are normally feed from separate fuses. Florescent fittings also often have their own fuse and any lights taken from the power circuit will normally also have a fused spur unit. Where BS 1362 fuses are used these should be 5 amp or less it is common to use 3 amp as it is the preferred size for low current equipment and most electricians carry them. Where high inrush causes a problem on switch on as with multiple spot lights on one circuit the B6 MCB may be changed for a C6 or D6 but before this is done the wiring will need testing to ensure it still gives required protection and is not a DIY job and should be notified under Part P as not replacing like for like in the consumer unit requires the issue of a completion certificate and this is fixed price for most local authorities so a registered electrician will do it for far less than the DIY cost. The use of grid switches allows fuses and switches in same unit. With the 17^th Edition lights are now being put on earth leakage protection devices and it seems may houses have borrowed neutrals on the stairs lighting circuits. With this in mind the only safe way to work on the house electrics is to switch off at main isolator the MCB is not always good enough.

Regulations
I have tried to give wiring regulation numbers in the main so where there are changes these can be verified. Part J, Part L, Part M and Part P may also control what is permitted. The wiring regulations alone is a 389 page book which then needs further books to explain what the contains mean. While writing this I knew there was a regulation to say the earth wire had to follow the same route as power cables but it took quite a time to locate 543.6.1 and so we get the Chinese whisper syndrome where people think there is a regulation but have never read it and jump to the wrong conclusions. I feel sure there will be one on minimum light levels for example but I have not seen one. In the main common sense must be used. The 17th Edition (BS 7671:2008) requires earth or RCD protection for wires buried in a wall where SELV (Separated Extra Low Voltage) this can not really be complied with so no further protection is used.
Where Part P requires that inspecting there seems some confusion as to whom should do this. It seems that the designer and or installer may need to complete the first set of results and only if the Local Authority Building control (LABC) wants to confirm the results do they need to complete a inspection report at their expense to verify your results. Since most DIY'er with not have the £700 worth of test equipment required it is unlikely they will be able to complete the first inspection report page 11 of the Part P document tries to explain who should do the testing but is full of contradictions and in fact seems to change area to area so only way it to ask your own LABC. Bathrooms require IPXXB or IP2X for SELV or IPX4 or if water jets are likely IP5X for 230 volt appliances within zone 2 the symbols should be found on any lights used under 2.25m and within the area of bath or shower tray. 

Energy saving
There has been a lot of miss information as to how to save energy. A house is heated either by radiation or convection the former is very hard to control whether as open coal fire or quartz electric. With convection however a room thermostat or radiator TRV can control the temperature with ease. To build up data on how much a bulb heats the house would need accurate air change information and even then because one feels warmer with radiated heat the air temperature can be lower but this needs human intervention as very hard to measure.
Looking at Carbon emissions reduction target on the internet and filling in a questionnaire I did talk to a nice man called Steve who tells me energy saving bulbs do save money but where the yearly saving is published as £40 in real terms more like £14 but no real research has been carried out as to real savings. So in other words he may be wrong and they could in real terms cost more.
Add to this a whole host of other unknowns like what is really inside an energy saving bulb and can they compensate for voltage variations. Add to this the energy costs of making the bulbs and the environmental impact of the mercury within the fluorescent bulbs or tubes leaves it very open as to if they really save any energy when used in doors. Because they as so effected by the cold not all types can be used out doors either.
The fluorescent comes in to basic types those using conventional chokes and starters and those with electronic control gear known a HF for High Frequency. The conventional chokes type is very voltage dependent and for a 230 volt unit at 215 volt it will likely fail and at 250 volt be using twice the ratted power. They rely on a capacitor to give power factor correction and if this fails it can use a third more power without any outward sign of any problem. The HF unit however has a very wide voltage range without any increase in power used often will run on DC as well as AC and can have battery back-up added tubes last much longer and maintain their output better and although they use same tubes can cost up to a third less to run. The lamp shown to the right under emergency lights below will be a HF unit I fitted a second hand unit at the top of my stairs in 1992 and I have never changed the tube to date 16 years latter and is in daily use.
With the compact fluorescent however there is little or no information on how they work and short of dismantling or installing power monitoring equipment one has no idea what type they are.
However the maintenance benefits and fire and burning risk is proved and there are many places it does make sense. In a kitchen under cupboards lighting a counter where it is already hot and also any tungsten bulbs could easy be touched and burn someone the discharge bulbs or LED lighting make sense.
It may also make sense where the heating is not thermostatically controlled or where a high number of lamps are used for decorative effect.
There are web sites with energy saving calculators but it will be noted it does not ask where the bulb is used and all seem to be on web sites of people selling bulbs! And some bulbs are very expensive I paid over £7 for the cold cathode GU10 bulb above my bed where a tungsten halogen one are two or more for a £1 but it is cold so I will not burn myself if I reach up and move it. 

Emergency lights
These come in two main types non maintained, maintained the maintained type can be used like an ordinary lamp. The non maintained only switch on with a power failure. One problem is where discharge lighting is used often a timer is included to hold the emergency light on when power returns to give time for main lights to strike. But this is hardly required in domestic use and they start at about £15 some torches are available which when on their charging base act as non maintained lights. In commercial premises a key switch would be added to test the lights but again at home switching off lighting MCB is good enough to test. The one to left is also a smoke alarm and looks far better than the standard type shown to the right. With the 17th Edition rules on taking account of danger that may arise from the failure of a single circuit such as a lighting circuit 314.1(iii) the use of emergency lights will be balanced out cost wise with cost of RCBO's so are likely to become more popular in the future.

Light dimming
With OSRAM DULUX EL VARIO energy-saving light can be varied without the need for dimmers: the integrated 2-stage dimmer function provides bright or subtle lighting simply by switching on and off. But in most cases we use dimming switches. OSRAM DULUX EL DIM is a new infinitely dimmable energy-saving lamp, which can used with appropriate lamp dimmers for all applications. There are others but most seem to tell about them but not sell and the dimmer needs to be 6 times bigger than the bulb size, also my web search failed to find prices, normal energy saving bulbs can not be dimmed. The Extra Low Voltage lamps also have problems with dimming trailing-edge phase dimmers and leading-edge phase dimmers (the latter normal for inductive loads) and trailing-edge phase dimmers

Notes
Fluorescent lamp starters normally come in ranges 4 - 22, 4 - 65, 4 - 80, 70 - 125 watt and also types electronic and standard for example FS - U in the main they are interchangeable but where two tubes are used i.e. fly catchers then the wrong type number may fail there is also an issue with the tubes some have earth strips down length and are of different thicknesses often they can be inter changed but sometimes they will fail the same with wattage a 2 ft tube can be 20 or 40 watt and again sometimes one can get away with wrong tubes but if you want long service it is better to ensure the correct tube and starter is used. Also remember they can't be disposed of in general rubbish best way is return them to shop where they can dispose of them correctly.
The same applies to extra low voltage spot lights using LED and lower wattage normally does not cause a problem but the inverters often have a min as well as max wattage and in some cases using LED lamps will fail and of course using larger lamps will burn out the transformer/inverter some more expensive inverters have an auto protection device built in but many don't and burn out.
It is common when changing lights to just shove a connection block up the hole where the wires come out of the ceiling if then one returns to these lamps one is often unaware of the out of sight wires which may when disturbed break a connection. Also the practice may be dangerous when other work is done from above. It is not possible to cover all aspects and however hard one tries to avoid dangers accidents will still happen. If your not sure ask, if your still not sure either ask again or call in an electrician and never work alone. Do isolate before working and remember both line and neutral are live so better to turn off power with double pole isolator than single pole MCB or RCBO and remember if you don't turn off at isolator and you have RCD's then touching neutral can both give a shock and switch off RCD so log off (switch off) computers etc before working. Also Wellingtons often contain graphite in the rubber mix to stop static build up so wearing Wellingtons and the like will not always protect you from shocks the gloves and goggles worn by supply authority employees to work on live supplies are special and only real safe way is turn off supply at main switch.    

Since originally making this page a number of questions have been asked. One big problem seems to be where an RCD is added either because lights are in a bathroom or the cables are buried less than 50mm in a wall. The Electrical Safety Council do give some guidance as to new regulations and where RCD's must be added although they do seem to miss out the option of using Ali-tube cable.
In this circuit the power and return are both coming from the same fuse and both meters show the same current with this the earth leakage trip will hold in and X1 the landing lamp can be switched off and on with the right hand switch also both the other lights can also be switched on and off without any problem.

There are other drawings on the page showing two way wiring and it can be seen how one would normally use triple and earth cable. But houses wired in 1980's often only had a single circuit for lighting and when the split between two floors first started electricians continued to wire in the old way. Later it was found this caused interference where the line and neutral did not follow the same route three cores started to be used. Although technically not to regulations because of borrowed neutrals until lights started to be put on RCD protection it was not seen as a problem and as a result quite common.

When reading any USA explanations they use the word "hot" where we use "line" be very careful with the word "live" as this refers to both line and neutral but some people in error refer to the "Line" and being "Live" which can cause confusion. And I have from time to time slipped up and used the wrong word.