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 a abbreviation however is actually a trademark, which belongs to Intel.
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.
Relocation of the memory and the CPU creating better ventilation and easier upgrade
Power management possible with proper BIOS support.

Micro ATX - A smaller version of Full ATX
Flex ATX - Another version of the ATX motherboard

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 this form factor is gaining popularity the last couple of years because there found on most clone computers
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

herboaagram 2

ATX Motherboard Diagram

A. Audio Ports
B. USB Ports
C. Firewire Ports
D. RJ45/Ethernet Ports
E. TV Tuner
F. PS2 Mouse and Keyboard Ports
G. P4 Socket
H. Cooling Tube Pipes
I. LGA CPU Socket
J. Memory (DIMM Slots)
K. Floppy Drive Controller
L. ATX 24pin Socket
M. Serial ATA Sockets
N. Northbridge Chipset
O. Southbridge Chipset
P. IDE Controller
Q. PCI-express X1
R. AGP
S. PCI
T. PCI-express X16
U. CMOS Battery
V. BIOS
W. Front Panel Connecto