X11 is one of the oldest protocol for remote graphical user interface (GUI). An X11 server runs on a computer that is capable of graphical display, while an X11 client can run on any computer. There are several X11 servers, but most Linux distributions now use Xorg as the defacto standard X11 server. X11 clients include all sorts of programs, such as Firefox (Iceweasel in Debian), OpenOffice, the GIMP and etc.