A mother board's primary purpose is to house the CPU and allow all devices to communicate with it and with each other. The two most popular motherboards are the Older AT, and the newer ATX.
(From A+ Guide to Hardware)
Dont think this above post answered the question correctly.....
There are:
AT
ATX
Mini-ATX
Micro-ATX
LPX
NLX
BTX
From what i was told