A GUI (or Graphical User Interface) is a visual display of information and user manipulatable controls on a display device. This is opposed to a textural user interface that can only display plain text. Textural user interfaces usually require the user to type in commands for all interactivity. GUIs usually allow for both keyboard entry as well as mouse user input. Newer GUIs are now supporting touch and even multi-touch interfaces.
Anyone using a modern operating system should be familiar with a GUI. It’s all those windows, buttons, menus and such that make up your computer’s desktop interface. This is true whether you’re using Mac, Windows, Linux, TV Game Console (like XBox 360, Playstation, or Nintendo), or even a smart phone.
Operating system makers like to name their GUIs. For example, the Mac OSX GUI is called “Aqua”. The Windows Vista and Windows 7 GUI is called “Aero Glass”. On Linux, there are many competing GUIs, but the two most popular are Gnome and GTK. To the average user, most of the user interfaces look very similar because they all provide the same, basic components (windows and controls like buttons, lists, radio buttons, drop down list boxes, etc…).