The term really only makes sense in the context of systems running the X Window System and/or UNIX, systems which have a GUI, but which may not have bundled all the modern features mentioned above. One can think of a desktop environment as a set of tools that make UNIX and X act like the GUIs of Microsoft Windows or Apple Macintosh.
Well-known examples include GNOME, KDE, CDE and XFce; however, a number of other desktop environments also exist, including (but not limited to): EDE, UDE, ROX Desktop, GEM, PerlTop, XPde, Xito and arm0nia.
Some window managers also include elements reminiscient of those found in desktop environments; most prominently Enlightenment.