Verified answer

Imagine you could take a regular ATX motherboard, and chop three slots off the end, making it shorter. That's mATX. It's basically the same motherboard, but shortened on one end so it has fewer slots. The remaining screw holes line up in the same positions, etc.

All ATX cases should accept an mATX motherboard. The difference will be that the shorter mATX motherboard will leave a couple slot positions unused.