This package contains xfwm4 which is scheduled to be the window manager of the Xfce project, version 4. Therefore, it is designed to run with the Xfce panel. Of course, you can run xfwm4 also with any other WM or even without one.
Install xfwm4 if you want a small, resource saving window manager which complies to the standards defined at http://www.freedesktop.org/. But be warned: If you install and run xfwm4 standalone, you won't be able to do anything since it doesn't even have a menu. Thus, we recommend to install xfce4 additionally.