A system unit is the metal case that houses and protects other computer components, such as the motherboard, hard drive, RAM, graphics card and central processing unit, or CPU, of a computer system.
Computer system units come in a number of different case types, with the most popular being rectangular desktop or mid-tower designs. They also contain a power supply unit, or PSU, which provides electrical current for the computer's components. The power supply also powers the fans, or a liquid cooling system, which prevents the computer from overheating. The individual components within a system unit are attached together using either data or power cables. External or peripheral devices are connected through a variety of data ports, such as USB, PS2, or IEEE 1394.