The motherboard is the main component found in PC and Macintosh computers. The motherboard is what allows various hardware components to transfer information to each other. As computers advanced, so did motherboards; below is a listing of the various Motherboard form factors.

Full-ATBaby-ATLPXFull-ATXMini-ATXNLX
Full-AT (12" wide x 13.8" deep) Matches the original IBM AT motherboard design, which only fits into full size AT or tower cases only, not being produced much any more, if any.
This form factor is no longer produced because it cannot be placed into the popular Baby-AT chassis.

Baby-AT (8.57" wide x 13.04" deep) Almost the same as the original IBM XT motherboard with modifications in the screw hole position to fit into AT style case, with connections built onto the motherboard to fit the holes in the case.

Specific placement of the keyboard and the I/O slots.
This board also cannot be placed into the slimline case.
LPX (9.00" wide x 13.00" deep) Developed by Western Digital when making motherboards, which was duplicated by many other manufacturers and is no longer made by Western Digital.
The LPX motherboard riser card contains all of the expansion slots.

Placement of the video, parallel, two serial and PS/2 connections have changed locations.
Full-ATX – (12" wide x 9.6" deep) / Mini-ATX – (11.2" wide x 8.2" deep) The official specifications were released by Intel in 1995 and was revised to version 2.01 in February 1997.

The ATX form factor is an advancement over previous AT style motherboards. Therefore requires a new case design. ATX is not an abbreviation, it is actually a trademark which belongs to Intel.

The ATX motherboard has a stacked I/O connector panel mounted on the motherboard.
On a socket 7 ATX motherboard, the socket has been placed a further distance from the expansion slots, allowing for long boards to be placed in easier.

Single keyed internal power supply connector. This is the Molex power connector, ATX 2.01. Standby voltage needs to be greater than 720 mA.

The connector now cannot be placed in improperly. While the Molex power connector allows for 5v and 3.3v to be connected, it is recommended that only a 3.3v be connected to the motherboard.

Relocation of the memory and the CPU creating better ventilation and easier upgrade.
Power management possible with proper BIOS support.

NLX (Supports motherboards with overall dimensions of 9.0" x 13.6" [maximum] to 8.0" x 10.0" [minimum]) Implemented in 1998 by Intel and is similar to the LPX form factor; however, includes several new improvements.

Support for the Pentium II
Support for AGP
Support for USB.
Support for DIMM.
Easier Access to internal components
Support for motherboards that can be removed without using tools.