Well Server is made for just that... using it as a server, for example a web server, if you run IIS with Pro, you can only have max 10 connection to the comp, while with server i think it's unlimited (but either way it's more). Also, there are more server use programs, I think. It's also been enhanced to run with networks. However, I've heard software such as games run better on Pro, as Server is meant more so for business.
--Mark
Correct me if I'm wrong on any of the above info.