OS X Server, formerly Mac OS X Server, was a separately sold Unix server operating system from Apple Inc. architecturally identical to its desktop counterpart Mac 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 Server App, an add-on package for OS X sold through the Mac App Store along with Server Admin Tools 10.7, available from the Apple support web site.
These tools simplify access to key network services, including a mail transfer agent, AFP and SMB servers, an LDAP server, a domain name server, and others. Also included (particularly in later versions) are numerous additional services and the tools to manage them, such as web server, wiki server, chat server, calendar server, and many others.
Read more about OS X Server: Overview, Server Administrator Tools, System Requirements, Technical Specifications, Languages