They are mainly window managers, that is their primary interface, but they also launch programs and show status, so I think of them as fat window managers and because they tend to be the only environment on the desktop, they are thin desktop environments.

Compare cwm with xfwm. while cwm is brutally minimalist in appearance it has enough extra functionality to be used as it's own desktop environment, xfwm requires several separate parts to be usable on the desktop.