The motherboard is the main component inside the case. It is a large rectangular board with integrated circuitry that connects the rest of the parts of the computer including the CPU, the RAM, the disk drives (CD, DVD, hard disk, or any others) as well as any peripherals connected via the ports or the expansion slots.
Components directly attached to the motherboard include:
The central processing unit (CPU) performs most of the calculations which enable a computer to function, and is sometimes referred to as the "brain" of the computer. It is usually cooled by a heat sink and fan.
The chip set mediates communication between the CPU and the other components of the system, including main memory.
RAM (Random Access Memory) stores all running processes (applications) and the current running OS.
The BIOS includes boot firmware and power management. The Basic Input Output System tasks are handled by operating system drivers.
Internal Buses connect the CPU to various internal components and to expansion cards for graphics and sound.
Current
The north bridge memory controller, for RAM and PCI Express
PCI Express, for expansion cards such as graphics and physics processors, and high-end network interfaces
PCI, for other expansion cards
SATA, for disk drives
Obsolete
ATA (superseded by SATA)
AGP (superseded by PCI Express)
VLB VESA Local Bus (superseded by AGP)
ISA (expansion card slot format obsolete in PCs, but still used in industrial computers)
External Bus Controllers support ports for external peripherals. These ports may be controlled directly by the south bridge I/O controller or based on expansion cards attached to the motherboard through the PCI bus.
USB
FireWire
eSATA
SCSI