The most popular Linux desktop environments: GNOME and KDE. Others include Cinnamon, XFCE, etc.
Desktop Environment is not a must for Linux, e.g. Ubuntu Server edition ships without a desktop. Desktop is just another software running in Linux, and it can be swapped, e.g. Ubuntu has several other flavors with different desktop environments: Kubuntu uses KDE, Xubuntu uses XFCE, etc.
- X11, a.k.a. X Window System, provides a basic framework for GUI. X protocol had been version 11 since 1987, thus X11.
- X.org is a popular open source implementation of X11.
- XDG stands for X Development Group, the old name of FreeDesktop.org.
- FreeDesktop.org is part of X.org Foundation.
- Wayland is intended as a simpler replacement for X.
- GNOME and KDE are expected to be ported to it.