The structure of a vehicle provides it with strength and is the platform to which the rest of the components are mounted. There are two types of vehicle structures commonly used: frame (or body on frame) and unibody.
Frame (body on frame)
A framed car has a frame that runs the length of the vehicle that usually consists of two side rails and various cross members that connect them at different points. The suspension, axles and engine are all mounted to the frame forming the chassis, and the body of the vehicle is mounted on top. At one time all cars used this form of construction. It is still the method used for some cars and most trucks today.
Unibody construction is when there is no separate frame from the body of the vehicle. The body is usually made of aluminum or steel that is stamped with the appropriate cross members and everything is mounted directly to it. This is the most common type of structure used today in passenger vehicles. It lowers the weight of the vehicle and distributes force away from the passengers in the event of a crash.
Both types of vehicle construction work well. When you buy a used car, you or your mechanic will be looking for different signs of structural damage, depending on the way the car is built.