The basic graphical display server X.Org, which is the official reference implementation of the X Window System. Alternatives include Wayland.
The X Window System, also called X11, X.Org, or X, is distributed as free software by its developer, the X.Org Foundation. It provides the basic framework for a GUI by drawing and moving windows and handling input from mouse, keyboard and touchscreen. The actual user interface is provided by client software.