Mac OS X Server will be the Server edition of the upcoming Mac OS X.
Yes, you may indeed say "Er, isn't Mac OS X Server out already?". The thing is now, that Apple has apparently wiped the Mac OS X Server as it were off the face of the earth, pretended that it had never been released, and is now due shortly after the release of Mac OS X.

The new Mac OS X Server will have the same beautiful Aqua interface as Mac OS X does. The main difference between the two is that Mac OS X Server is meant as a dedicated server OS, thus not for everyday workstation use.

Since our dearest Steve Jobs has somewhat neglected to tell anybody at all about the secret replacement of the entire product, there's little to tell about it at the moment, except for the info that Apple carries on their website.

Noone has seen any beta versions around of this one, nor has anyone seen the OS really run.

Therefore I'll just redirect the lot of you to Apple's Mac OS X Server page.