When buying a cases for a new PC you are building, what specifications do you look for to check that the motherboard will fit into the case properly? Are all the screw holes in standard places?
It is called "form factor." ATX is one form factor, there's also micro-ATX, and BTX which isn't very popular now. When your looking at cases, it'll tell you whether it'll support ATX, micro-ATX, or both.