Xcode - The Gui Wonder

Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. (formerly Apple Computer, Inc.) for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface. The original form of what Apple would later name the "Mac OS" was the integral and unnamed system software first introduced in 1984 with the original Macintosh, usually referred to simply as the System software.

Early versions of the Mac OS were compatible only with Motorola 68000-based Macintoshes. As Apple introduced computers with PowerPC hardware, the OS was ported to support this architecture as well. Mac OS 8.1 was the last version that could run on a "68K" processor (the 68040). Mac OS X, which has superseded the "Classic" Mac OS, is compatible with both PowerPC and Intel processors through to version 10.5 ("Leopard"). Versions 10.6 ("Snow Leopard") and later support only Intel processors.

The early Macintosh operating system initially consisted of two pieces of software, called "System" and "Finder", each with its own version number. System 7.5.1 was the first to include the Mac OS logo (a variation on the original Happy Mac startup icon), and Mac OS 7.6 was the first to be named "Mac OS".

Before the introduction of the later PowerPC G3-based systems, significant parts of the system were stored in physical ROM on the motherboard. The initial purpose of this was to avoid using up the limited storage of floppy disks on system support, given that the early Macs had no hard disk. (Only one model of Mac was ever actually bootable using the ROM alone, the 1991 Mac Classic model.) This architecture also allowed for a completely graphical OS interface at the lowest level without the need for a text-only console or command-line mode. Boot time errors, such as finding no functioning disk drives, were communicated to the user graphically, usually with an icon or the distinctive Chicago bitmap font and a Chime of Death or a series of beeps. This was in contrast to computers of the time, which displayed such messages in a mono-spaced font on a black background, and required the use of the keyboard, not a mouse, for input. To provide such niceties at a low level, Mac OS depended on core system software in ROM on the motherboard, a fact that later helped to ensure that only Apple computers or licensed clones (with the copyright-protected ROMs from Apple) could run Mac OS.

Mac OS X is the newest of Apple Inc.'s Mac OS line of operating systems. Although it is officially designated as simply "version 10" of the Mac OS, it has a history largely independent of the earlier Mac OS releases.

The operating system is the successor to Mac OS 9 and the "classic" Mac OS. It is a Unix operating system, based on the NeXTSTEP operating system and the Mach kernel which Apple acquired after purchasing NeXT Computer, with its CEO Steve Jobs returning to Apple at this time. Mac OS X also makes use of the BSD code base. There have been six significant releases of the client version, the most recent being Mac OS X 10.7, referred to as Lion. On Apple's October 20th 2010 "Back to the Mac" event, Mac OS X 10.7 Lion was previewed, showing improvements and additions including a Mac App Store.

As well as the client versions, Mac OS X has also had six significant releases as a server version, called Mac OS X Server. The first of these, Mac OS X Server 1.0, was released in beta in 1999. The server versions are architecturally identical to the client versions, with the differentiation found in their inclusion of tools for server management, including tools for managing Mac OS X-based workgroups, mail servers, and web servers, amongst other tools. It was the default operating system for Xserve (which has now been discontinued), it's an optional feature on the Mac Mini and the Mac Pro, and it's also installable on most other Macs. Unlike the client version, Mac OS X Server can be run in a virtual machine using emulation software such as Parallels Desktop and VMWare Fusion.