This article is from the MachTen & CodeBuilder FAQ, by Jaime Julca email@example.com with numerous contributions by others.
The X Window System is a protocol for performing windowing functions across
a network. "Windowing functions" means drawing lines, arcs and other
graphics primitives on any type of console or display. As a protocol, it is
implemented in a set of libraries freely available from the Massachusetts
Institute of Technology (MIT).
If an application supports the X Window protocol, the graphical output of
that application can be displayed on any X terminal. The application is
called an "X client" and the X terminal is considered an "X server".