Short for Graphical User Interface, a GUI (pronounced as either G-U-I or gooey) allows the use of icons or other visual indicators to interact with electronic devices, rather than using only text via the command line. For example, all versions of Microsoft Windows utilize a GUI, whereas MS-DOS does not. The GUI 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.
In the article over at Computer Hope, the GUI is explained in great detail. From the workings of GUI to examples and how does a user actually interact with a GUI.
A few notable examples of different GUIs are:
KDE (K Desktop Environment) GUI
Google Chrome GUI
Windows 10 GUI
Apart from a clear visual distinction between the examples, I was able to better understand the various interaction methods that users might use.
Typically the user uses a pointing device such as the mouse to interact and use most aspects of the GUI. However, it is also possible to interact with a GUI using a keyboard or other input device.