Formfactors.org

Plantwiz

http://www.formfactors.org/formfactor.asp

Read up on:
For information and specifications please see the following links:

Motherboard Form Factors
ATX
MicroATX
FlexATX
BTX

Power Supply Form Factors
ATX
TFX
SFX
Non Form Factor Specific
CFX
LFX

etc....