OS previously Mac OS X, is a series of Unix-based graphical interface operating systems developed, marketed, and sold by Apple Inc. It is designed to run exclusively on Mac computers, having been pre-loaded on all Macs since 2002. 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. The first version released was Mac OS X Server 1.0 in 1999, and a desktop version, Mac OS X v10.0 "Cheetah" followed on March 24, 2001. Releases of OS X are named after big cats: for example, OS X v10.8 is referred to as "Mountain Lion".

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. The 'X' is also used to emphasize the relatedness between OS X and UNIX. Versions 10.5 "Leopard" running on Intel processors, 10.6 "Snow Leopard", 10.7 "Lion" and 10.8 "Mountain Lion" have obtained UNIX 03 certification. iOS, which runs on the iPhone, iPod Touch, iPad, and the 2nd and 3rd generation Apple TV, shares the Darwin core and many frameworks with OS X. An unnamed variant of v10.4 powered the first generation Apple TV.

OS X originally ran on PowerPC-based Macs. In 2006, the first Intel Macs had a specialized version of 10.4 "Tiger". In 2007, 10.5 "Leopard" was the first to run on both PowerPC and Intel Macs with the use of Universal Binaries. 10.6 "Snow Leopard" was the first version of OS X to drop support for PowerPC Macs. Mac OS X 10.7 "Lion" was the first version of OS X to drop support for 32-bit Intel processors and run exclusively on 64-bit Intel CPUs.

The server edition, OS X Server, was architecturally identical to its desktop counterpart, and included tools to facilitate management of workgroups of OS X machines, and to provide network services. Starting with v10.7 "Lion", OS X Server is no longer offered as a separate operating system product; instead, the server management tools are available for purchase separately, and are preloaded on the server models of Mac Pro and Mac Mini along with OS X.
 
OS X is based upon the Mach kernel. Certain parts from FreeBSD's and NetBSD's implementation of Unix were incorporated in NeXTSTEP, the core of Mac OS X. NeXTSTEP was the graphical, object-oriented, and UNIX-based operating system developed by Steve Jobs' company NeXT after he left Apple in 1985. While Jobs was away from Apple, Apple tried to create a "next-generation" OS through the Taligent, Copland and Gershwin projects, with little success.

Eventually, NeXT's OS, then called OPENSTEP, was selected to be the basis for Apple's next OS, and Apple purchased NeXT outright. Steve Jobs returned to Apple as interim CEO, and later became CEO, shepherding the transformation of the programmer-friendly OPENSTEP into a system that would be adopted by Apple's primary market of home users and creative professionals. The project was first known as Rhapsody and was later renamed to Mac OS X.

Mac OS X Server 1.x, was incompatible with software designed for the original Mac OS and had no support for Apple's own IEEE 1394 (FireWire) interface. Mac OS X 10.x included more backward compatibility through Classic and more functionality by introducing the Carbon API as well as FireWire support. As the operating system evolved, it moved away from the legacy Mac OS to an emphasis on new "digital lifestyle" applications such as the iLife suite, enhanced business applications (iWork), and integrated home entertainment (the Front Row media center).[18] Each version also included modifications to the general interface, such as the brushed metal appearance added in version 10.3, the non-pinstriped titlebar appearance in version 10.4, and in 10.5 the removal of the previous brushed metal styles in favor of the "Unified" gradient window style.

In 2012, with the release of OS X Lion, the "Mac" prefix was officially dropped in all references to the operating system name within marketing materials - and with OS X Mountain Lion "Mac" was dropped in all references within the operating system itself.

Distribution and Languages

As of September 2011, OS X is the second-most-active general-purpose client operating system in use on the World Wide Web, (after Microsoft Windows), with an 8.45% usage share according to statistics compiled by W3Counter. It is the most successful Unix-like desktop operating system on the web, estimated at over 5 times the usage of Linux (which has 1.5%).

There are twenty-two "System Languages" available for the user at the moment of installation (the "system language" is the entire operating system environment). As of Mac OS X Lion, the languages are Arabic, Chinese (Simplified), Chinese (Traditional), Czech, Danish, Dutch, English, Finnish, French, German, Hebrew, Hungarian, Italian, Japanese, Korean, Norwegian, Polish, Portuguese (Brazilian), Portuguese (European), Russian, Spanish, Swedish and Turkish. Input methods for typing in dozens of scripts can be chosen independently of the system language.

OSx86

OSx86 (from Mac OS X and x86) is a collaborative hacking project to run the Mac OS X computer operating system on non-Apple personal computers with x86 architecture and x86-64 compatible processors. The effort started soon after the June 2005 Worldwide Developers Conference announcement that Apple would be transitioning its personal computers from PowerPC to Intel microprocessors.

A computer built to run this type of Mac OS X is also known as a Hackintosh, a portmanteau of the word "hack" and the name of Apple's main brand of computers, Macintosh. Hackintoshed notebook computers are also referred to as "Hackbooks".

The Apple software license does not allow Mac OS X to be used on a computer that is not "Apple-branded". The legality of this form of tying is disputed by companies such as Psystar, Bizon computer, PearC and MacPC who have attempted to release products using Mac OS on non Apple-machines. However, while the methods Apple uses to prevent Mac OS X from being installed on non-Apple hardware are protected from commercial circumvention in the United States by the DMCA, specific changes to the law regarding the concept of jailbreaking has thrown such and similar circumvention methods into a grey area when carried out by end-users for personal use.