Stands for "Graphical User Interface" and is pronounced "gooey." It is a user interface that includes graphical elements, such as Windows, icons and buttons. The term was created in the 1970s to distinguish graphical interfaces from text-based ones, such as command line interfaces. However, today nearly all digital interfaces are GUIs.
The first commercially available GUI, called "PARC," was developed by Xerox. It was used by the Xerox 8010 Information System, which was released in 1981. After Steve Jobs saw the interface during a tour at Xerox, he had his team at Apple develop an operating system with a similar design. Apple's GUI-based OS was included with the Macintosh, which was released in 1984. Microsoft released their first GUI-based OS, Windows 1.0, in 1985.
For several decades, GUIs were controlled exclusively by a mouse and a keyboard. While these types of input devices are sufficient for desktop computers, they do not work as well for mobile devices, such as smartphones and tablets. Therefore, mobile operating systems are designed to use a touchscreen interface. Many mobile devices can now be controlled by spoken commands as well.
Because there are now many types of digital devices available, GUIs must be designed for the appropriate type of input. For example, a desktop operating system, such as OS X, includes a menu bar and Windows with small icons that can be easily navigated using a mouse. A mobile OS, like iOS, includes larger icons and supports touch commands like swiping and pinching to zoom in or zoom out. Automotive interfaces are often designed to be controlled with knobs and buttons, and TV interfaces are built to work with a remote control. Regardless of the type of input, each of these interfaces are considered GUIs since they include graphical elements.
NOTE: Specialized GUIs that operate using speech recognition and motion detection are called natural user interfaces, or NUIs.