Also known as: X Window System · Xorg · X server
X11 is the long-standing display server protocol for Unix-like systems, providing a network-transparent client-server architecture in which applications connect to an X server that manages display output and input devices.
X11's network transparency was a genuine innovation, allowing applications to run on one machine and display on another — the mechanism behind SSH X forwarding.
Its security model reflects its 1980s origins: clients on the same display are mutually trusting and can read one another's windows and input. Combined with decades of accumulated protocol extensions, this is what motivated the move to Wayland, though X11 remains in use where its permissiveness is a feature.
Wayland
Wayland is a display server protocol for Linux designed to replace the X Window System, in which a single compositor handles rendering, input, and window management rather than the client-server split X uses.
Linux Distribution
A Linux distribution is a complete operating system built around the Linux kernel, bundling it with system libraries, a package manager, default utilities, and a curated set of software, along with policies for how all of it is maintained and updated.
Open Source
Open source software is software distributed under a licence that grants anyone the right to use, inspect, modify, and redistribute the source code, subject to conditions defined by that licence.