Short for Graphical User Interface, the GUI (pronounced as either G-U-I or gooey) was first developed at Xerox PARC by Alan Kay, Douglas Engelbart, and a group of other researchers in 1981. Later Apple introduced the Lisa computer, the first commercially available computer on January 19, 1983. A GUI uses windows, icons, and menus to carry out commands such as opening files, deleting files, moving files, etc. and although many GUI Operating Systems are operated by using a mouse, the keyboard can also be used by using keyboard shortcuts or arrow keys.
Unlike a command line operating system or CUI like Unix or MS-DOS, GUI Operating Systems are much easier for end-users to learn and use because commands do not need to be known or memorized. Because of their ease of use, GUI Operating Systems have become the dominant operating system used by end-users today.