A graphical user interface is a human-computer interface that is graphical (rather than purely textual) user interface. GUI uses images, window, icons, buttons, menus etc. which can be manipulated by a mouse.
GUI are more user friendly than command line interface(CLI) as user interacts by using a mouse rather than by having to type in commands. Also users don’t have to remember lot of commands and thus it becomes easier for the user to learn and use the system. GUI is often pronounced as “Gooey”.
Today’s major operating systems provide a graphical user interface. Although GUI are more user friendly, they are not as flexible and robust as CLI. For example, multiple CLI commands can be combined using pipes to perform tasks that would be much more cumbersome to perform with GUI programs and no GUI tool can replace file attribute-matching schemes of find command(a UNIX command) to locate files.