I assume that you're talking about motherboard form factor. If that's correct, form factor means the physical characteristics of motherboards.
It specifies the size of the board, where the mounting holes are located, where the ISA, PCI and AGP slots are and in which direction they are facing, what type of power supply connector, and so on. This standardisation makes it possible for motherboards, power supplies, cabinets and expansion cards from various manufacturers to be compatible with each other.
There are various types, the most common ones for personal desktop PCs are -
AT - mainly obsolete now. They were used in older motherboards such as those for Pentium, some Pentium II and III, and their equivalents. Usually has 5 or 6 slots.
Baby AT - as above except that it's vertical size is shorter and has fewer expansion slots.
ATX - currently the most popular one along with micro-ATX. Used for P3, P4, Athlon XP, AMD 64, etc. Usually has 5 or 6 PCI slots and an AGP slot.
Micro-ATX - as above except that it's vertical size is shorter and has fewer expansion slots - usually 2 or 3 PCI slots. May or may not have an AGP slot. Very popular especially for economy mobos with onboard graphics.
There are other types, and some branded computers use their own non-standard form factors. BTX has been recently introduced; it's supposed to be an improvement over the ATX standard.
