Abbreviated UI, user interface is the means by which a user and a computer system interact. It is a set of commands or menus through which a user communicates with a program. In other words (UI) is everything designed into an information device with which a human being may interact. Examples may include keyboard, mouse, display screen, light pen, etc. It determines how commands are given to the computer or the program and how information is displayed on the screen.
A command-driven interface is one in which you enter commands. A menu driven interface is one in which you select command choices from various menus displayed on the screen.
The user interface can arguably include the total "user experience," which may include the aesthetic appearance of the device, response time, and the content that is presented to the user within the context of the interface. In early computers UI was limited.
UI is one of the most important parts of any program because it because it determines how easily you can make the program do what you want. A powerful program with a poorly designed user interface has little value. Graphical user interfaces (GUIs) that use windows, icons, and pop-up menus have become standard on personal computers.