GUI--Graphical User Interface.  Most computer programs started out as text-based interfaces in which the user would type a command and the program would respond with some action.  A GUI uses graphics objects like windows, buttons, and dialogue boxes to communicate the user's commands to the program, with the goal of making the process easier to learn and use, hence, more efficient from the user's point of view.  Of course, the interface itself requires computing resources, so it is not necessarily efficient from the computer's point of view!