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System Unit

The system unit is the plastic or metallic box that sits under your monitor or desk
and is covered with slots, buttons, and lights.

Anything outside of and connected to the system unit is called a peripheral. You
can add dozens of peripherals and accessories to make it more useful and fun.
Common peripherals include printers, scanners, external hard drives, CD-ROM
drives, and digital cameras. Many peripherals are considered to be input or
output devices.

Inside the system unit are these parts illustrated in the picture below:

Labeled System Unit

NB:

Motherboard: The main piece of circuitry in a computer. Everything
connects to or is wired to the motherboard.
Central Processing Unit (CPU): The computer’s brain or heart, the
CPU is a computer’s main chip. The CPU does all the work: It
calculates, it processes, it keeps things running smoothly. A CPU’s
speed is measured in megahertz (MHz) and gigahertz (GHz). Common
CPUs include: Intel Pentium III, Intel Pentium 4, etc