OS X Server was a server operating system distributed by Apple Inc. which was architecturally identical to its desktop counterpart OS X, with additional functionality.
OSX-Server is a computer with Server Operating System similar to Desktop Operating System with additional administration, management and additional server programs for management purpose of several group of computers. For more technical detail please visit technical specification
As of version 10.7, Mac OS X and Mac OS X Server were combined into one release and re-branded as OS X. A separate "server" operating system is no longer sold; the server-specific server applications and work group management and administration software tools from Mac OS X Server are now offered as OS X Server, an add-on package for OS X sold through the Mac App Store