"UI" is short for "User Interface." The user interface is what governs the interaction between a person and a machine. In the world of computing, user interfaces fall into various categories with the most common ones being graphical user interfaces (GUI), command line interfaces and touch-screen interfaces.

Back in the early days of personal computers, command-line interfaces were dominant. The original Microsoft DOS operating system is a classic example of an operating system which uses a command line interface. Far from being user-friendly when compared to today's user interfaces, a command line interface made it necessary for users to type in a command to carry out various tasks or launch programs. Command line interfaces are still in use, however. Windows 7, for example, provides a command line interface which you can access by typing "cmd" into the start menu search box and pressing Enter. This feature is also available in Windows 8 and will certainly not be going away any time soon.

These days, for the most part, users interact with their computers through a graphical user interface. The first Microsoft operating system to use a GUI was Windows 1.0, introduced in 1985 as a graphical addition to Microsoft DOS. Graphical user interfaces are far more user-friendly as well as being more pleasant to look at. It also makes multitasking (running multiple programs simultaneously) easier. All of today's consumer operating systems provide GUIs.

Touch-screen user interfaces are a form of graphical user interface designed primarily for touch-screen interaction rather than keyboard and mouse input. Many higher-end mobile phones and other mobile devices such as tablet PCs have touch-screen interfaces. Windows 8, due to be released in late 2012, focuses greatly on touch-screen interaction with its new touch-optimized "Metro" interface.

User interfaces come in many more different forms. For example, the popular gaming console, the Nintendo WII, provides a gesture-driven interface, monitoring the way the player moves. Windows Vista and above provide a speech-based user interface as well, allowing you to control the computer simply by using your voice.