X11
also: X Window System, X, Xorg
X11 is a network-based graphical display system that manages windows, input devices, and graphics rendering on Unix and Linux systems. It follows a client-server architecture where applications (clients) send drawing requests to a display server.
X11 (also called X Window System) is the standard display protocol for Unix-like systems. It uses a client-server model: your applications are clients that communicate with an X server, which controls the actual monitor, keyboard, and mouse. This separation allows you to run graphical apps remotely over a network.
For example, you can SSH into a remote Linux machine with ssh -X user@host and run a graphical application like gedit or firefox, and the window will appear on your local screen while the program executes remotely.
X11 has been the foundation of Linux desktop environments like GNOME and KDE for decades. However, newer display servers like Wayland are emerging as modern replacements that offer better performance and security.