MicroATX, it reduced the cost of ATX type motherboards by removing the amount of expansion slots reducing the need for a big power supply allowing the motherbard to fit in smaller cases.
MicroATX
There are many motherboard sizes, and ATX is simply one of them. Others inlude Mini-ITX, MicroATX, etc.
ATX is the form factor.
ATX
There are no similar form factors to ATX if you're talking about a slimline. Slimline form factors are NLX and LPX. The form factors to ATX are BTX, Micro-ATX, and Flex-ATX. There is no correlation between slimline and ATX.
ATX is the form factor which Motherboards, Computer Cases and Graphics Cards use to standardise sizing. An ATX form-factor Motherboard will fit an ATX Case, and an ATX Graphics Card will fit in the case as well.
MB is ATX form factor.
ATX
It depends on the type of board you have. Obviously a laptop mobo (motherboard) is going to be smaller than one in a desktop. And again a desktop one is going to be different than one for a server. Motherboards are categorised by a thing called form factor. So for example the form factor of an ATX board is going to be different than the form factor of an extended ATX. ATX would be the standard board you would use in a desktop PC. Whereas you might use mini-ATX for one of those new mini computers you see, or you might use extended-ATX in a server. So when someone asks what form factor the board is, they're really just asking "is it atx or micro-atx or mini-atx etc..." And obviously each form factor is going to be different in size. There are some very good online tutorials about mobo's, check google.
Before the ATX form factor was released, XT, AT, and BabyAT were the most common standard form factors.
atx,nlx,btx
ATX.