The document discusses the components and functions of a computer system unit. It describes the system unit as the enclosure that houses the computer's electronic components. It lists common components inside the system unit including input/output ports, the power button, reset button, motherboard, power supply, disk drives, and hard disk drive. The motherboard is described as the main circuit board that contains additional components like the central processing unit, RAM, ROM, and expansion slots. RAM is used for temporary storage while ROM contains permanent, unchangeable programs from the manufacturer. Expansion slots allow additional cards to be inserted to expand the computer's capabilities.