Mini ATX (ITX) Mini ATX, Micro ATX, ATX, Full ATX, EATX
(Smallest standardized board at the moment, fits in all ATX chassis, one PCIe slot, Some ITX chassis have no expansion slots, so the slot can't be used) Generally two expansion slots are provided in the chassis so larger GPUs can be used.
uATX (MicroATX) Micro ATX, ATX, Full ATX, EATX
Commonly found in off the shelf OEM computers, generally the cheapest boards available. 4 PCI slots. Most Chassis will have 4 expansion slots, occasionally 5. It is very common for these chassis to have half height slots that will not accept larger cards.
ATX ATX, Full ATX, EATX
Original ATX standard. Commonly 7 PCI slots, most chassis will have 7 expansions slots. Some will offer 8.
Full ATX Full ATX, EATX
Usually the same 'Height' as a standard ATX board, but wider. Most Full ATX cases offer a lot of extra general room.
E ATX E ATX
EATX boards and chassis have up to 11/12 expansion slots. Not too common.
(There are custom and enterprise class boards out there, rarely come up though)