X Window System is a legacy (1984) windowing system, that is a system providing the basic framework for a GUI environment initially developed as part of Project Athena at MIT. The currently available X protocol version is 11 (dated 1987!) - hence often people refer to X Window as "X11''.
It is really time I wanted to write a few posts on this amazing software that really made the story of IT on the UNIX and Linux platforms: it is one of my favorite software ever, it was really impressive in those years being able to run an application on a host while being able to just render on a remote display.
In the X window - remote display manager (XDMCP) and thin clients we see how to reproduce on a modern Linux distribution a very common setup of the late '80 and early '90: a remote server running applications rendered on X Window based thin clients.