A motherboard is the central or primary circuit board making up a computer system or other complex electronic system. A typical computer is built with the microprocessor, main memory, and other basic components on the motherboard. Other components of the computer such as external storage, control circuits for video display and sound, and peripheral devices are attached to the motherboard via connectors or cables of some sort.
There is more discussion of motherboards in IBM compatible personal computers in PC motherboard.
See also Front side bus, BIOS