A chassis is a framework which supports an inanimate object, analogous to an animal's skeleton; eg in an automobile or aeroplane.
A computer's chassis is the rigid framework onto which the motherboard, memory, disk drives and other equipement are mounted. It also supports the shell-like case: the housing that protects all of the vital internal equipment from dust, moisture, and tampering. The artistic styling of the otherwise rather functional and plain computer encasings is called case modding.