Size is the advantage.
MicroATX is usually much smaller than normal ATX and usually has fewer expansion slots like PCI (+variants), fewer DIMM sockets (usually 1 maybe 2) they are often built around budget or low cost CHIPSETS. Much of this depends on the manufacturer. Some motherboards come with the CPU soldered on while others use a socket. Often mATX boards cost the same as ATX and if space is not an issue in your chasis, then go with a Standard ATX. Standard ATX provides a better upgrade path over µATX.
ATX is just a way of describing the type of motherboard.
There are not really any advantages with a ATX over a Micro ATX just Micro ATX is smaller and ATX is larger
adding and removing components easier, better support for I/O devises and processor technology, lowering costs.
ATX
flex ATX above that is the micro ATX
I have often wondered what ATX in ATX software stood for. I always just ignored it but upon further inspection I have discovered that the ATX stands for Advanced Technology Extended.
ATX improves upon AT by making adding and removing components easier, providing greater support for I/O devices and processor technology, and lowering cost.
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.
Asus is simply a manufacturer that makes atx and non-atx motherboards. Whether a motherboard is ATX or not is specific to that single model.
Yes, if it is an ATX motherboard. The motherboard specifications should say specifically if it is ATX, mini ATX, BTX, ITX, etc. Most motherboards are 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.
usually any motherboard that has a form factor above mini or micro atx (atx, extended atx, super atx) will have atleast 4 if not more expansion slots available by default and normally an atx (and possibly a extended atx) motherboard should fit in a full tower case (a super atx motherboard will more than likely need a super tower)
Baby ATx and ATx