Like any other computer system, all of the basic circuitry and components required for a PC to function are either contained in or attached to the motherboard. A PC motherboard allows the attachment of the CPU, [Video Card]?, [Sound Card]?, IDE 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 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 Pro, and Super7 (Socket 7 with a 100MHz bus speed) is for AMD K6, K6-2, and K6-3 processors (I reckon our server runs a Pentium III on a socket 7 but I'd have to check --drj).
There are usually a number of expansion card slots to allow peripheral devices and cards to be inserted. Each slot will be compatible with one or more industry bus standards. The commonly available busses are: ISA (Industry Standard Architecture), EISA (extended ISA), MCA (micro channel architecture), PCI (peripheral component interconnect), AGP. An AGP slot is a high speed bus designed solely for connecting high performance [graphic cards]? (which produce video output) to the PC. ISA was the original bus for connecting cards to a PC, it was superseded by the (compatible) EISA bus. PCI is faster than ISA and EISA and is largely replacing EISA. MCA was designed by IBM for its PS/2 series of computers and is now (in 2001) largely obsolete and rarely found.
As of 2001 a typical motherboard might have 1 AGP slot, 4 PCI slots, and 1 (or 2) EISA slots. EISA slots will exist because of a large number of already existing peripheral cards which can only be plugged into an EISA (or ISA) slot.
Some of the other devices found in a typical PC used to be installed on cards which themselves were inserted into a PCs expansion slots: The IDE controller (to access IDE hard disks), serial ports (COM ports), parallel ports (printer ports). As of 2001 most of those devices are found 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. The form factors mainly seen today are ATX, Mini-ATX, and (coming soon) NLX.
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