The ones that matter most to PC builders and upgraders are ATX, MicroATX, and Mini-ITX. ATX is sometimes referred to as "standard ATX," and ATX boards (usually, but not exclusively) measure 9.6x12 inches. They're what you'll see in most midtower or larger PC cases—what most of us think of as traditional tower PCs. A few multi-CPU boards, intended for servers and workstations, and some outliers (such as EVGA's Classified series boards) support larger ATX "standards" such as Extended ATX and XL-ATX, but these won't be of interest to most PC upgraders or builders. The key thing to know apart from the size factor: ATX boards will have more expansion slots than MicroATX or Mini-ITX ones.
