Bicycles/Equipment and Accessories/Lights
Lights are an essential cycling accessory and any cyclist that intends to ride in dim lighting conditions, or poor weather such as fog or rain should not consider riding without lights.
Why use lights? 
Although bikes sold for use on the roads are required to be supplied with reflectors, these really should not be relied on to keep you visible on the roads . Cyclists should be responsible for their own safety and acknowledge that not only are proper lights a common courtesy to other road users, they are one of the ways to promote your safety on the roads. Darkness and bad weather can make cyclists almost invisible to other road users. A light may be the only thing that makes a driver or pedestrian realize that the faint shadow up ahead is actually a cyclist.
Types of Lighting 
There is a wide selection of bicycle lights available, all with their own characteristics. It is definitely worth discussing your needs and cycling habits with the staff in the store you intend to buy your lights from.
There are generally three types of lighting used for bicycles, based around either Light Emitting Diodes (LEDs), Halogen filament bulbs or High-Intensity Discharge (HID) bulbs. It is very rare to see lights based around other light sources, so unless they are expressly recommended by an expert they should be treated with caution when buying. Bicycle lights can also be powered by different means. The most common are ordinary batteries, rechargeable batteries, "bottle" dynamos and Hub dynamos.
LED lighting systems 
LED lights are very popular amongst cyclists and have a number of excellent characteristics. They are lightweight, relatively cheap, bright, compact and reliable. They require very little maintenance- LEDs have a very long lifespan and don't need to be replaced like bulbs. LEDs are generally battery powered and give excellent battery life. Many LED lighting systems are also demountable; you can detach the lights and take them with you if you are going to leave your bike locked up in public.
LED lights often come with the option of a variety of flashing modes, as well as a steady beam. While you are required by law to have lights attached to the frame that emit a steady beam, many cyclists also attach additional lights to their bike or clothing that are set to flash. Flashing lights tend to attract the attention of drivers and increase battery life
One issue with LEDs is that they emit quite a narrow beam that appears weaker the further you move from the optimum viewing angle. All LED lights should have a light diffuser which will increase their effectiveness.
Halogen lighting systems 
Most cyclists find that LEDs do not provide a powerful enough beam to illuminate the road. This means that while they work very well as a tail light in all conditions and as a headlight in areas with street lighting they will not provide illumination of the way ahead like a car's headlights might. Cyclists that need to cycle down unlit roads often choose a Halogen light for their headlight. Halogen lights give a very bright beam for relatively low power consumption, although they are more power hungry than LED systems. They tend to be available with more power source options than LED lights, such as rechargeable batteries and dynamos. Because they use more batteries and produce more heat than LEDs, halogen lights also tend to be very slightly bigger and heavier than LEDs, the trade off being their superior performance for specialist use.
Power Sources 
Bicycle lights are normally powered by either batteries or dynamos. The Key difference between batteries and dynamos is that, by using a dynamo you generate your own electricity to power the lights on your bike and by using batteries you get power from a disposable (and environmentally harmful) source. Both the lights and the generators for Dynamo systems are initially more expensive than battery powered lights, however this can be offset over the lifespan of the product by the fact that you are not paying to recharge or replace batteries. While most battery powered lights attach to the bike by a simple clip mechanism, meaning they can be removed to prevent theft, most dynamo lights have to remain clipped to the frame due to the wiring that connects them to the Dynamo.
Battery power= 
LED systems tend to run off battery power, as it provides a steady, constant, compact and lightweight source of power that suits the characteristics of LED systems. LEDs don't use battery power very quickly and are a very economical form of bicycle lighting. You can use your own rechargeable batteries but they tend not to come as standard
Increasing your visibility 
Always check that the lights you buy are weatherproof. You will need them in the rain!
Links with more information 
http://en.wikipedia.org/wiki/Bicycle_lighting Wikipedia entry on bicycle lighting http://www.runmuki.com/commute/ a guide to bicycle communing http://www.lcc.org.uk/ the London Cycling Campaign website