A Desktop Environment is hard to define without referring to itself. Something that provides a unified look and feel (as a WindowManager would) but on top of that, a desktop environment provides set of tools for managing logins/sessions/applications and perhaps even a toolkit for creating other applications. MicrosoftWindows' desktop environment is Explorer.
For Linux, there are two obvious contenders in this field:
The concept dates back to Sun's CDE, the Common Desktop Environment. It was the graphical environment that Solaris was based on. It includes the login manager, the session manager, the window manager, and various desktop tools.