WHAT IS Mother Board ?

Motherboard is the main circuit board of a microcomputer. 

AT / ATX DIFFERENCES 

The following is some of the ways in determining if your motherboard is an AT motherboard or an ATX motherboard.

The Keyboard:
AT Motherboard = DIN/5 connector

ATX Motherboard = PS/2 connector

Motherboard power connector
AT Motherboard = Single Row two connectors 5v & 12v
ATX Motherboard = Double row single connector 5v, 12v, and 3.3v
In this section, you learned that the motherboard holds all the components of the computer that serves as their main attachment point. 

What components are found on a typical motherboard

CPU type: CPU socket or CPU slot
Memory slots: SIMM slots, DIMM slots or RIMM slots
Cache memory: Internal or External Cache found on the CPU and as support chips
Chipsets: Northbridge, Southbridge, Cache, Heatsink, MCC, sound and video chipsets
System BIOS: ROM usually a DIPP chip and complemented by the CMOS battery
Expansion slots: AGP, ISA, PCI, AMR and CNR
Motherboard connectors: FDC, IDE Controllers, Fan Controllers, CD/DVD and sound controllers
Motherboard settings: Jumpers and Switches
Power connectors: AT socket vs. ATX socket

Types of motherboards

(old school)
AT (Full vs. Baby)
XT (rip)
LPX (rip)

(newer)
ATX
NLX

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 

 A. Primary and Secondary IIDE Controllers
B. ROM/BIOS
C. ISA slots
D. CMOS Battery
E. PCI slots
F. DIN/5 Keyboard Connector
G. AT Socket
H. ATX Socket
I. DIMM Slots
J. SIMM Slots
K. Chipset
L. L2 Cache
M.CPU Socket
N. Floppy Drive Controller
O.LPT Connector
P.COM Connector

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

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 Connectors

Another Motherboard