The graphical user interface (GUI) was actually developed by Xerox and introduced on its Star workstation in 1981. Apple borrowed heavily from the Star, and subsequently, others copied the Mac, moving the GUI down the line to Windows, OS/2 and UNIX. This is the operating system that runs on Macintosh computers. It is pronounced, "mack-oh-es." The Mac OS has been around since the first Macintosh was introduced in 1984. Since then, it has been continually updated and many new features have been added to it. Each major OS release is signified by a new number (i.e. Mac OS 8, Mac OS 9). Since the core of the Mac OS was nearly decades old, Apple decided to completely revamp the operating system. In March of 2001, Apple introduced a completely new version of the Mac OS that was written from the ground up.

The company dubbed it "Mac OS X," correctly pronounced "Mac OS 10." Unlike earlier versions of the Mac OS, Mac OS X is based on the same kernel as Unix and has many advanced administrative features and utilities. Though the operating system is much more advanced than earlier versions of the Mac OS, it still has the same ease-of-use that people have come to expect from Apple software. The Macintosh interface was immediately popular with non-technical people. Instead of typing in a command to delete a file as in DOS, you could drag it to the on-screen trashcan. Although common today, it was a breakthrough to have such capability on a personal computer in the 1980s. Unlike the PC, the Mac is Apple's proprietary technology, and except for a brief period, Apple prevented a Macintosh clone industry from developing and growing (see Macintosh parts). Apple maintained its sole source vendor status while the PC industry had thousands of vendors.