A server edition of the Mac OS.
Mac OS server, based on BSD Unix-based operating system software Apple acquired from Next Computer formed the basis of the current OS X.
Prior to OSX, Apple's Appleshare services provided server applications such as file and printer sharing and user management.
As of version 10 of the MacOS, the server and the standard version are nearly identical; OSX Server mainly adds server management software - web, mail and user services. Most of OSX Server's services are based on standard BSD tools, for example the Apache web server; OSXS adds a GUI interface to these services. An exception is User services which are based on (Next) NetInfo.
See Mac OS.