X11 (aka X, because older versions haven't been relevant in decades) is a protocol/architecture for GUIs. The X.Org Foundation is a group that maintains the Xor

X11 (aka X, because older versions haven't been relevant in decades) is a protocol/architecture for GUIs. The X.Org Foundation is a group that maintains the Xorg server (the engine that actually draws graphics somewhere) as well as several X protocol client libraries (used by applications), other server implementations, and associated tools. libX11 is the primary client library. https://magcius.github.io/xplain/article/x-basics.html https://magcius.github.io/xplain/article/x-basics.html is probably a good place to start, as is the official documentation (indexed at https://www.x.org/releases/current/doc/index.html https://www.x.org/releases/current/doc/index.html , but the stuff at the top may not be what you want - you probably want to start with the protocol docs https://www.x.org/releases/current/doc/xproto/x11protocol.html https://www.x.org/releases/current/doc/xproto/x11protocol.ht... or perhaps the libX11 docs https://www.x.org/releases/current/doc/libX11/libX11/libX11.html https://www.x.org/releases/current/doc/libX11/libX11/libX11.... ).