The Graphical User Interface (GUI)
The GUI had its roots in the 1950s but was not developed until the 1970s when a group at the Xerox Palo Alto Research Center (PARC) developed the Alto, a GUI-based computer. The Alto was the size of a large desk, and Xerox believed it unmarketable. Jobs took a tour of PARC in 1979, and saw the future of personal computing in the Alto. Although much of the Interface of both the Lisa and the Mac was based (at least intellectually) heavily on the work done at PARC, and many of the engineers there later left to join Apple, much of the Mac OS was written before Job's visit to PARC. When Jobs accused Bill Gates of Microsoft of stealing the GUI from Apple and using it in Windows 1.0, Gates fired back:
No, Steve, I think its more like we both have a rich neighbor named Xerox, and you broke in to steal the TV set, and you found out I'd been there first, and you said. "Hey that's no fair! I wanted to steal the TV set!
The fact that both Apple and Microsoft had gotten the idea of the GUI from Xerox put a major dent in Apple's lawsuit against Microsoft over the GUI several years later. Although much of The Mac OS is original, it was similar enough to the old Alto GUI to make a "look and feel" suit against Microsoft dubious.
For a more detailed discussion of the origins of the Mac OS GUI, click here