Defines the type of processor and RAM the motherboard requires and determines to a degree the built-in devices the motherboard supports, including the expansion slots. The built in components determine the core functionality of the system.
Determines the physical size of the motherboard as well as the general location of components and ports.
Invented by IBM in the early 1980s, was the predominant form factor for motherboards through the mid-1990s. AT is now obsolete.
Slimline form factor
Used to create PCs that were very thin. NLX and LPX were two examples of this form factor.
The first slimline form factor; replaced by NLX form factor.
Second form factor for slimline systems. Replaced the earlier LPX form factor. (NLX apparently stands for nothing; it's just a cool grouping of letters.)