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What is a Motherboard?
http://www.hardwaretipsandtricks.com/articles/12/1/What-is-a-Motherboard/Page1.html
By Super Admin
Published on 11/21/2006
 
A motherboard, also known as a mainboard, system board, or logic boards on Apple Computers, and sometimes abbreviated as mobo (generally credited to the magazine Maximum PC) is the central or primary circuit board making up a complex electronic system, such as a modern computer.

Motherboard

A typical computer is built with the microprocessor, main memory, and other basic components on the motherboard. Other components of the computer such as external storage, control circuits for video display and sound, and peripheral devices are typically attached to the motherboard via ribbon cables, other cables, and power connectors.

Historically, a computer was built in a case or Mainframe with a series of wired together connectors called a backplane into which the cpu, memory and I/O on separate cards was plugged. With the arrival of the microprocessor, it became more cost-effective to place the backplane connectors, processor and glue logic onto a single 'mother' board, and have the video, memory and I/O on 'child' cards - hence the terms 'Motherboard' and Daughterboard.

One of the first popular microcomputers to feature this design was the Apple 2 computer, which had a motherboard and 8 expansion slots.

There is more information about IBM-compatible personal computers in PC motherboard.

Connects to:
Devices via Cables
Chips via Sockets
Riser Cards via one of
PCI
AGP
PCI Express
 
Form Factors:
AT
ATX
microATX 

Form factors

Motherboards are available in a variety of form factors, which usually correspond to a variety of case sizes. The following is a summary of some of the more popular PC motherboard sizes available:
 
PC/XT - the original open motherboard standard created by IBM for the first home computer, the IBM-PC. It created a large number of clone motherboards due to its open standard and therefore became the de facto standard.
AT form factor (Advanced Technology) - the first form factor to gain wide acceptance, successor to PC/XT. Also known as Full AT, it was popular during the 386 era. Now obsolete, it is superseded by ATX.
Baby AT - IBM's successor to the AT motherboard, it was functionally equivalent to the AT but gained popularity due to its significantly smaller physical size. It usually comes without AGP port.
ATX - the evolution of the Baby AT form factor, it is now the most popular form factor available today.
ETX, used in embedded systems and single board computers.
Mini-ATX - essentially the same as the ATX layout, but again, with a smaller footprint.
microATX - again, a miniaturization of the ATX layout. It is commonly used in the larger cube-style cases such as the Antec ARIA.
FlexATX - a subset of microATX allowing more flexible motherboard design, component positioning and shape.
LPX - based on a design by Western Digital, it allows for smaller cases based on the ATX motherboard by arranging the expansion cards in a riser (an expansion card in itself, attaching to the side of the motherboard - image). This design allows the cards to rest parallel to the motherboard as opposed to perpendicular to it. The LPX motherboard is generally only used by large 
 
Mini LPX - a smaller subset of the LPX specification.
NLX - a low-profile motherboard, again incorporating a riser, designed in order to keep up with market trends. NLX never gained much popularity.
BTX (Balanced Technology Extended) - a newer standard proposed by Intel as an eventual successor to ATX.
microBTX and picoBTX - smaller subsets of the BTX standard.
Mini-ITX - VIA's highly integrated small form factor motherboard, designed for uses including thin clients and set-top boxes.
WTX (Workstation Technology Extended) - a large motherboard (more so than ATX) designed for use with high-power workstations (usually featuring multiple processors or hard drives.
While most desktop computers use one of these motherboard form factors, laptop (notebook) computers generally use highly integrated, customized and miniaturized motherboards designed by the manufacturers. This is one of the reasons that notebook computers are difficult to upgrade and expensive to repair - often the failure of one integrated component requires the replacement of the entire motherboard, which is also more expensive than a regular motherboard due to the large number of integrated components in it.