Hey Colaboy
There are only 2 common forms of motherboards the AT and the ATX.
But there was also the Baby AT. And theres a Mini ATX ....
The AT were introduced in 1984 measured around 12"x13" and supported 80286 or the older cpu's. They also had the 5.25 " floppy not commonly seen today which is the 3.5". There was also a Baby AT motherboard which is still being used today by some companies. The ATX was introduced I think around 1996 by Intel and is the most commonly used form in newer pcs.
Heres some you can read.
and