Chassis

The chassis, also often called the "case", is the container or enclosure that holds all of the other pieces of a computer. It might include some switches, fans, and indicator lights. Desktops and servers will each use a chassis of a different size and shape, but the functions are similar.

Empty server chassis:

Empty desktop chassis:

Motherboard

The motherboard is a circuit board containing slots or sockets for the CPU and memory, and also contains supplementary circuitry. The motherboard will often have networking, video, and audio components built in, and it will usually have expansion slots for additional components to be added or upgraded. Motherboards are mounted in the chassis, typically in the largest open space you see in these pictures. The other open spaces are for other components, like fans, hard drives, and optical drives.

Server motherboards are similar to desktop motherboards, but usually are larger, and are capable of containing more CPUs and more RAM.

Server motherboard:

Desktop motherboard: