Graphical user interface commonly called GUI (pronounced Gooey) is the means of building code to a visual display of icons, widgets, pointing device, windows, skin, menu-based interface. Prior to a desktop GUI, changes to software were made by command-line. This limited the reach of computers to only people educated in computer science. The GUI dramatically improved ease of use to the point that children up to the elderly could now operate a computer. The most popular GUI became Windows by Microsoft. Other GUI's include Apple's OSx, O/S 2 from IBM. For Linux you have a choice of GNOME, KDE, LXDE, Unity, Xfce and others. UNIX has the X Windows GUI.
The graphical user interface is not limited to computers and can be found on electronic devices that require human-interaction such as a phone, a car, touchscreens, etc. The first GUI was developed by Xerox PARC in 1973. It ran on their Alto personal computer.