A motherboard is a printed circuit board used in a personal computer. It is also known as the mainboard and occasionally abbreviated to mobo. The term "mainboard" is also used for the main circuit board in other electronic devices.
The remainder of this article discusses the so-called "IBM compatible PC" motherboard.
Like any other computer system, all of the basic circuitry and components required for a PC to function sit either directly on the motherboard or on an expansion card slided in a slot of the motherboard. A PC motherboard allows the attachment of the CPU, graphics card, Sound card, IDE/ATA/Serial ATA Hard disk Controller, Memory (RAM), and almost all the other devices in the computer system. It contains the chipset, which controls the operation of the CPU, PCI, ISA, and AGP expansion slots, and (usually) the IDE/ATA controller as well. Most of the devices that can be attached to a motherboard are attached via one or more slots or sockets.
There are different slots and sockets for CPUs according to what CPU you want to use, it's important that the motherboard has the right socket for the CPU. Socket A is used for AMD Athlon and Duron processors, Slot A is for older AMD Athlon processors, Socket 478 is for the Pentium 4 Northwood processors, Socket 423 is used for Intel Pentium 4 processors, Socket 370 is for Intel Pentium III and Celeron processors, Slot 1/Slot 2 is for older Intel Pentium II/III and Celeron processors, Socket 7 is for Intel Pentium and Pentium MMX, Super7 (Socket 7 with a 100MHz bus speed) is for AMD K6, K6-2, and K6-3 processors, and Socket 8 is for Pentium Pro. The newer sockets with a three digit number is named after the number of pins it contains. The older ones are simply named after their order of invention.
There are usually a number of expansion card slots to allow peripheral devices and cards to be inserted. Each slot is compatible with one or more industry bus standards. Commonly available busses include: ISA (Industry Standard Architecture), EISA (extended ISA), MCA (Micro Channel Architecture), VESA (Video Electronic Standards Association), PCI (Peripheral Component Interconnect), and AGP (Advanced Graphics Port). ISA was the original bus for connecting cards to a PC; despite significant performance limitations it was not superseded by the more advanced but incompatible MCA (IBM's proprietary solution which appeared in that firm's PS/2 series of computers and a handful of other makes)or the equally advanced and backward-compatible EISA bus, but endured as standard in new PCs till the end of the 20th century, aided first by the brief dominance of the VESA extension during the reign of the 486, and then by the need to accommodate the large number of existing ISA peripheral cards. The more recent PCI bus is the current industry standard, which initially was a high-speed supplement to ISA for high-bandwidth peripherals (notably graphics cards, network cards, and SCSI host adaptors), and gradually replaced ISA as a general-purpose bus. An AGP slot is a high speed, single-purpose port designed solely for connecting high performance graphics cards (which produce video output) to the PC.
As of 1999 a typical motherboard might have had 1 AGP slot, 4 PCI slots, and 1 (or 2) ISA slots; since about 2002 the last ISA slots in new boards have been replaced with extra PCI slots.
Some of the other devices found in a typical PC used to be installed on expansion cards which themselves were inserted into a PCs expansion slots: The IDE controller (to access IDE hard disks), serial ports (COM ports), parallel ports (Centronics/printer ports). Since about 1994, most of those devices have usually been integrated into the motherboard (which frees up some expansion slots).
As of 2001 most PCs also support Universal Serial Bus (USB) connections; again, USB support is usually integrated into the motherboard. An ethernet card is also commonly integrated into the motherboard, although not as commonly as the other devices mentioned.
The motherboard fits into the computer case with screws or clips. There are many "Form-Factors," or sizes of motherboard, so if you are planning to buy a new one, make sure it will fit the specifications for the case you have.