TL;DR; X11 (and X windows) is a standard and a protocol. X is a short term for X11 and X Windows Xorg is a specific implementation of X11 protocol under the GN

TL;DR; X11 (and X windows) is a standard and a protocol. X is a short term for X11 and X Windows Xorg is a specific implementation of X11 protocol under the GNU open source project. Pretty much every UNIX under the sun had some kind of X11 implementation, until proprietary UNIX workstations became a thing of the past.