Also see From AT to BTX: Motherboard Form Factors  in Webopedia's "Did You Know...?" section.

Motherboard form factors
Describes the various form factors for PC motherboards, with separate sections on the Baby-AT, ATX, and LPX form factors. This page is from "The PC Guide".

PC system case reference guide
A complete look at the PC system case, including its major components, different sizes and styles, form factors, switches, LEDs and drive bays.

System Case Form Factors
Describes the different form factors used for PC cases, with special attention to the "Baby AT" and ATX form factors. This page is part of "The PC Guide".