Steel and Aluminum
Most cars intended for mass production and consumer use have bodies made from either steel or aluminum. Both are strong metals, but steel is cheaper than aluminum. Aluminum, however, is lighter and does not rust, and so is used for more expensive luxury and performance models than steel.
Plastic
Plastics are being used more frequently in modern car manufacturing. Cars like the Chrysler CCV are made with all plastic bodies with a mind to fuel economy, as well as recycling when the car comes to the end of its workable life. Plastic is also easy to work, and repairs to plastic body panels are cheap.
Carbon Fiber
Carbon fiber is very light and very strong, meaning that fast cars can get the best out of their engines. It is also expensive, however, and so is primarily used in high-end sports and racing cars.