Actually theres seven: in order from largest to smallest.
ATX Full Tower
Small form factor (or Flex, sometimes called shuttle, but Shuttle is actually a pioneering manufacturer of them)
And theres three differences in actual hardware.
ATX fits in ATX full, mid and desktop
Micro atx fits in all micro cases
Flex atx fit in Micro and SSF (I think)
System specs? Lets just get down to it people:
My penis is bigger than yours.