http://netbsd.i2p/docs/guide/en/chap-x.html
These are suites of integrated software designed to give you
a well-defined range of software and a more or less common interface
to each program. These typically include a window manager, file
manager, web browser, email client, multimedia player, text editor,
address book, help browser, etc. As you may have guessed, a desktop
environment is not needed to use X, but many users will want to
install one.