For the most part it's size, and AT is very old computer wise. See here for more detail:
http://www.devhardware.com/forums/motherboards-32/at-vs-atx-179846.html
The main differences between ATX and Mini-ITX motherboards are their size and expansion capabilities. ATX motherboards are larger, typically measuring 305 x 244 mm, and offer more slots for RAM, PCIe cards, and additional features, making them suitable for high-performance builds. In contrast, Mini-ITX motherboards are smaller, measuring 170 x 170 mm, which limits the number of expansion slots and RAM capacity, but allows for more compact, space-saving builds. This makes Mini-ITX ideal for small form factor PCs, while ATX is better for more extensive setups.
ATX Motherboards are often the preferred motherboard to use in a desktop computer. According to TechRadar, some of the best manufacturers of the Micro ATX motherboards are ASUS, AMD, MSI and Sapphire.
That all depends. Most Z270 Motherboards will fit.. the case supports ATX, Micro ATX, and Mini ITX form factor boards. But you will not be able to fit E-ATX motherboards.
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.
NLX has slots parallel to the motherboard as ATX does not
Atx, btx, nlx
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, MicroATX, Flex ATX, BTX, and NLX, In that order.
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.
Most ATX motherboards offer a choice of several similar processors. The manufacturer decides which processors can used by a specific ATX motherboard. ATX is a physical and electrical specification. ATX does not define a specific central processor, so theoretically an ATX motherboard could be created for any type or brand of processor that can function within the specification. Common ATX motherboards use either an Intel processor or an AMD processor.
The four most popular motherboards types on the market today include ATX, Micro ATX, Baby AT, and proprietary boards. Proprietary motherboards are created by large PC manufacturers such as Dell, IBM, and Compaq, and are sold as part of their PCs.