The system unit, also referred to as a "tower" or "chassis," houses the desktop computer's essential parts. The processor, CPU, Memory, and other parts are included. The system unit also includes the container that holds these components.

The main element is distinct from peripheral components such as the display, keypad, and mouse. A "workstation" is created by peripherals working with the main unit.

Some contemporary computers, like the iMac, integrate the main unit and display into one unit. The display in this instance is a component of the system device. Although notebooks also have built-in screens, since the word "system unit" only applies to desktop computers, they are not regarded as system units.