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