Within the ATX specification, there are different sizes and layouts. ATX mostly means that it takes a certain kind of power supply, and that the keyboard/mouse/etc ports are arranged a certain way. Also, it has its mounting points in certain locations. But beyond that, it's going to vary in other specifications.
Whether or not it fits in your case is going to depend on the case, the position of the drive bays and power supply, and the position of the RAM chips and the CPU on the motherboard. These things all vary.
Once, I had to use a dremel tool to cut away a portion of the disk drive mounting cage in an old case when I added a new motherboard. Couldn't get the RAM chips in because the new motherboard put them under the drive cage.