The headlight lens is one component that makes up the headlamp assembly. It is the clear outer piece that the light shines through with the help of a reflector to light up the road. It is often available as a separate piece which saves you the costs or having to buy the entire assembly if there is damage to it. The headlight lens can easily break in an accident which will expose the bulb and other parts of the assembly to wide and rain. It can also oxidized or clouded over which reduces the effectiveness of the lights. Have a clear intact lens is also required to pace inspection in each state. A cracked, yellowed or fogged over lens also looks bad reducing the value and appeal of your vehicle.
Headlights have been in use and the lens is important to protect the light from debris and the elements. They are almost always clear and made out of plastic or glass. Since the headlamps are often thought of as the "eyes" of the car, they are designed in many different shapes and give the car its distinctive look and personality. The headlight lens may also cover up fog lights or turn signals. http://en.wikipedia.org/wiki/Headlamp
Ready to replace the headlight lens? You're in luck because it is one of the easiest jobs to do on a car. These are instructions on how to change the lens that are generic in nature and not a substitute for an official shop manual or a trained mechanic. As always, make sure that you have the correct parts and tolls to complete the job.