I'm building a desktop and I'm going to get the SilverStone SST-ST50F 500W as my PSU. What form-factor is it?
Is it ATX, following the motherboard form-factor or different?
"ATX" when used with a PSU refers to the power and signaling specifications.
Standard PSUs have had the same form factor for a very long time, all the way back to the AT PSU days. The only difference is the number and arrangement of power pins, and the use of a physical switch versus soft-signaling.
And yes, the PSU you name should work with a ATX motherboard.