A window manager will only manage your windows, so you have to rely on other
applications such as nm-applet to connect to the internet, PCManFM for file
management, etc.
Desktop environments contain a lot more than a window manager, for example in
Trisquel we've got a bottom panel (gnome-panel), a wallpaper that we can
easily change, icons on the desktop, important applications (such as file
manager).
This probably explains the difference better than I do:
http://everyjoe.com/technology/explain-whats-the-difference-between-a-window-manager-and-a-desktop-environment/
