The chassis of a car is the frame that the engine sits in, and the body covers. This frame needs to be durable enough to withstand the vehicle's weight in addition to G-forces and potential collisions.
There are many types of chassis designs, some of which are suited for a particular type of vehicle (such as a work truck or race car). Sometimes it is possible to upgrade the rigidity of a chassis with a bolt-on part or by welding in support structures. The chassis tends to include the wheels (often distinguished by the term "rolling chassis"), and include components which are both sprung and unsprung weight. The suspension system is also part of the chassis, part of the unsprung weight, and the part of the chassis which moves the most, reducing the flexing within the frame.
Many cars have a chassis/body which is ground for the electrical system, which reduces the amount of wiring needed for simple electronic components.