A motherboard (sometimes alternatively known as the mainboard, system board, planar board or logic board) is a printed circuit board (PCB) found in all modern computers which holds many of the crucial components of the system, such as the central processing unit (CPU) and memory, and provides connectors for other peripherals.

Motherboard specifically refers to a PCB with expansion capability - the board is the "mother" of all components attached to it, which often include sound cards, video cards, network cards, extra hard drives or other forms of persistent storage, TV tuner cards, cards providing extra USB or Firewire slots, and a variety of thousands of other kinds of custom components. (The term mainboard is applied to devices with a single board and no additional expansions or capability - in modern terms this would include controlling boards in televisions, washing machines and other embedded systems, which are not true motherboards.)

Read more about Motherboard:  History, Design, Bootstrapping Using The BIOS