Linux desktop concepts defined

The X Window System (commonly X11, or X) is a networking and display protocol which provides windowing on bitmap displays. It provides the standard toolkit and protocol to build graphical user interfaces (GUIs).

Window Managers (WM) control the placement and appearance of application windows in conjunction with the X Window System. Types of WM:

Desktop Environments (DE) work atop and in conjunction with X, to provide a completely functional and dynamic GUI. A DE typically provides a window manager, icons, applets, windows, toolbars, folders, wallpapers, a suite of applications and abilities like drag and drop. Examples.

Source: The great archlinux Wiki :-)