Advanced technology extended
A full ATX offers more flexibility than a Micro ATX or a Mini ATX.
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)
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.
ATX
Advanced Technology Extended Switched Mode Power Supply
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.
It's an ATX Full tower Chassis and all ATX form factor power supplies should work with it.
The most popular form factors are: -Mini ITX -Micro ATX -ATX -E ATX
The three form factors for desktop system boards are: ATX, and BTX and NLX
Some motherboard form factors (from smallest to largest in physical size) are: -Mini ITX -Micro ATX -ATX -E ATX
Before the ATX form factor was released, XT, AT, and BabyAT were the most common standard form factors.