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.

The X window system (commonly X Window System or X11, based on its current major version being 11) is a computer software system and network protocol that provides a basis for graphical user interfaces (GUIs) and rich input device capability for networked computers. It creates a hardware abstraction layer where software is written to use a generalized set of commands, allowing for device independence and reuse of programs on any computer that implements X.

X originated at the Massachusetts Institute of Technology (MIT) in 1984. The current protocol version, X11, appeared in September 1987. The X.Org Foundation leads the X project, with the current reference implementation, X.Org Server, available as free and open source software under the MIT License and similar permissive licenses.

X is an architecture-independent system for remote graphical user interfaces and rich input device capabilities which allows many people to share the processing power of a time-sharing computer and to collaborate with each other through client applications running on remote computers. Each person using a networked terminal has the ability to interact with the display with any type of user input device. Due to the ubiquity of support for X software on Unix, Linux and Mac OS X, X is commonly used to run client applications on personal computers even when there is no need for time-sharing.

OS X ( /oʊ ˌɛs ˈtɛn/), formerly Mac OS X, is a series of Unix-based graphical interface operating systems developed, marketed, and sold by Apple Inc. OS X (officially) runs exclusively on Macintosh computers and has been pre-loaded on all Macs since 2002.

OS X, whose X is the Roman numeral for 10 and is a prominent part of its brand identity, is built on technologies developed at NeXT between the second half of the 1980s and Apple's purchase of the company in late 1996. It was the successor to Mac OS 9, released in 1999, the final release of the "classic" Mac OS, which had been Apple's primary operating system since 1984.

OS X is a UNIX-like operating system that originally ran on PowerPC-based Macs. In 2006, the first Intel Macs had a specialized version of Mac OS X v10.4 "Tiger". in 2007, Mac OS X 10.5 "Leopard", was the first to have UNIX 03 certification and run on both PowerPC and Intel Macs with the use of Universal Binaries. Mac OS X 10.6 "Snow Leopard" was the first version of OS X to drop support for PowerPC Macs and run solely on Intel's x86-based processors. Mac OS X 10.7 "Lion" was the first version of OS X to drop support for 32-bit processors and run exclusively on 64-bit CPUs.