motherboard or central printed circuit board (PCB)

 a motherboard is the central printed circuit board (PCB) in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple computers, the logic board. It is also sometimes casually shortened to mobo.
The motherboard serves to connect all of the parts of a computer together. The CPU, memory, hard drives, optical drives, video card, sound card and other ports and expansion cards all connect to the motherboard directly or via cables.

The motherboard can be thought of as the "back bone" of the computer.

Modern motherboards include, at a minimum:

    sockets (or slots) in which one or more microprocessors may be installed
    slots into which the system's main memory is to be installed (typically in the form of DIMM modules containing DRAM chips)
    a chipset which forms an interface between the CPU's front-side bus, main memory, and peripheral buses
    non-volatile memory chips (usually Flash ROM in modern motherboards) containing the system's firmware or BIOS
    a clock generator which produces the system clock signal to synchronize the various components
    slots for expansion cards (these interface to the system via the buses supported by the chipset)
    power connectors, which receive electrical power from the computer power supply and distribute it to the CPU, chipset, main memory, and expansion cards.

modern budget motherboard for computers based on AMD processors, has on-board support for a very large range of peripherals:

    disk controllers for a floppy disk drive, up to 2 PATA drives, and up to 6 SATA drives (including RAID 0/1 support)
    integrated graphics controller supporting 2D and 3D graphics, with VGA and TV output
    integrated sound card supporting 8-channel (7.1) audio and S/PDIF output
    Fast Ethernet network controller for 10/100 Mbit networking
    USB 2.0 controller supporting up to 12 USB ports
    IrDA controller for infrared data communication (e.g. with an IrDA-enabled cellular phone or printer)
    temperature, voltage, and fan-speed sensors that allow software to monitor the health of computer components