Standard ATX, same as the standard one in the picture in here:
https://en.wikipedia.org/wiki/ATX
Chrisss: Potentially, yes. Assuming it's a PCIe/NVMe capable slot, you can get much faster transfer rates, lower seek times etc. SATA is limited to 6Gb/s (600MB/s in other words).
Note that M2 is just a form factor and can run in legacy SATA mode, however it can also support PCIe/NVMe mode which allows for transfer rates of many GB/s. The Samsung 960 Evo drive in my machine for example is specced for up to 3.2GB/s read transfer rate and 1.9GB/s write.
I doubt I get that kind of performance out of it, but the point is that for modern SSDs, SATA is the bottleneck and NVMe/M.2 removes that bottleneck.
Edit: Just ran the Samsung Magician benchmark on my drive. Scores are 2,598 MB/s read and 1,758MB/s write. Certainly a lot more than SATA can deliver...
EDITED: 28 Aug 2018 16:11 by DAVE!!