https://unix.stackexchange.com/questions/276168/what-is-x11-exactly
X11 is the 11th version of X. it has been the standard for the last 25 years. X11 is responsible for all wm and de guis. It was created by MIT and now maintained by the Xorg foundation. X runs as a server process where clients connect to to run the lower level code responsible for creating widgets.