X Server

The X server accepts requests from client applications to create windows, which are (normally rectangular) “virtual screens” that the client program can draw into. Windows are then composed on the actual screen by the X server (or by a separate composite manager) as directed by the window manager, which usually communicates with the user via graphical controls such as buttons and draggable titlebars and borders. For a comprehensive overview of X Server and X Window System, consult the following article: https://en.wikipedia.org/wiki/X_server