What is a body kit? It is a collection of exterior modifications to a car, typically comprising front and rear bumpers (fenders), side skirts, and spoilers.
What is it made of? Kits are usually constructed of either fiberglass or polyurethane or with the latest material: PU Fibre (Which is softer and more resistant to knocks than fibreglass). Most car modification enthusiasts 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.
What about Ground Effects? Ground effects are parts of a vehicle's lower body designed to enclose the gap between the bottom of the fuselage and the ground. The purpose of ground effects are to deflect air around the car rather than under it. Air passing under a vehicle exerts a lifting effect at high speed, which reduces the tires' contact with the road, resulting in loss of control. Ground effects can be part of the vehicle's original design, or aftermarket parts added later.
Hopefully this information will help give you a basic understanding of body kits and ground effects.