Automative body kits are usually constructed of either fiberglass or polyurethane. Most car modification entusiasts prefer polyurethane because it is more resistant to damage, and does not crack in cold weather. In some cases, body kits may also be constructed of carbon fiber, which is a popular, lightweight material. This type of modification is not as highly regarded among speed enthusiasts, though, because it is very costly, and the difference in weight is minimal, versus changing other parts such as the hood or trunk.
Factory fitted bodykits are now becoming more common, perhaps in response to the growth of the aftermarket tuning industry in the late nineties and onwards.
Guy2: Dude, It looks like a rice burner.
Player2: Fuck that shit!! I only drive stock!!
On the other hand there are body kits that are actually useful to the car and not ugly. These kinds are all mostly made by JDM Part companies such as TRD, Mazdaspeed, and Nismo. The body kits are produced by these companies to actually make use of aerodynamics while being able to preserve the car's beauty.
2) That supra with the TRD kit is damn sexy.