GUI is an acronym for Graphic User Interface. GUI is one of the benchmarks in the development of computer operations. The interface is essentially a program using the graphic coordinate grid as a series of switches with specific functions, depending on the set up of the computer screen or web page. This technology was a major breakthrough when it was developed, allowing much more flexibility in operation of computer screens, and far better interactions than the old line based system, which worked much like hyperlinks or search boxes.
Examples of GUI:
Web page functions
A screenshot of a modern GUI (GNOME), taken from ubuntu version 9.04 jaunty jackalope.