Bicycle lights are an absolute necessity If you intend to ride at night, or even at dusk. And,you should have both a headlight and a taillight.
There are some very clever designs to choose from. Many have features such as tool-free mounting and different lighting options such as different flash rates, different degrees of side-lighting, etc.
The lower cost lights use a series of AAA or AA, batteries and the battery life can range from 30 hours steady light up to 80 hours flashing light.
These are probably OK for a very occasional ride in diminished light. But, If you plan to ride, routinely, at night the lights with a rechargeable NIMH or lithium-ion battery are a much better option.
Rechargeable bike light
Some lights are fine for city cycling, where there is more ambient light, and are more useful for being seen than they are for helping you to see. Others are better for seeing on country roads where there is little ambient light.
In any case, their effect is limited at high speeds where you need to see an obstacle and react faster.
And, remember you could find yourself going much faster than usual on a downgrade.
There are lights that mount on your helmet that have the added advantage of enabling you to see whatever you are looking at. These are handy for unlocking your bike or finding stuff in your bag but you need to avoid looking directly at an oncoming driver.
The lower cost lights use a series of AAA or AA, batteries and the battery life can range from 30 hours steady light up to 80 hours flashing light. 
