Visual part of computer application or operating system through which a user interacts with a computer or a software. It determines how commands are given to the computer or the program and how information is displayed on the screen. Three main types of user interfaces are (1) Command language: the user must know the machine and program-specific instructions or codes. (2) Menus: user chooses the commands from lists displayed on the screen. (3) Graphical user interface (GUI): user gives commands by selecting and clicking on icons displayed on the screen.