OS X Server
OS X Server, formerly Mac OS X Server, is a separately sold Unix server operating system from Apple Inc. architecturally identical to its desktop counterpart OS X – with additional server programs and management and administration tools.
As of version 10.7 (Lion), 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 along with Workgroup Manager 10.8, available from the Apple support web site.