A car chassis may refer to either the frame of a car that holds together its components or to a rolling chassis. A rolling chassis consists of the frame, engine and drivetrain. That is, it includes almost all components except the body. Most modern automobiles are not built with a rolling chassis, as unibody construction is more common.
It is common for people to refer to the skeletal frame of a car as the chassis, although in some cases this may not be entirely accurate. For many vehicles made in the early-to-middle 20th century, the chassis was simply the car minus the body. Antique vehicles will generally be constructed this way, making the chassis easily identifiable. Later, a technique called monocoque was developed, in which parts of the body and frame were welded together to form a single unit.
Nearly all cars sold in the U.S. today are assembled by unibody construction. Unibody frames integrate the body and frame of the vehicle, which helps reduce weight, while maintaining a solid unit. With reduced weight, cars benefit from increased fuel economy, because the engine is not working as hard. However, many pickup trucks, vans and SUVs still use a body-on-frame design to maximize towing weight and payload.