The Xfce 4 Window Manager is part of the Xfce Desktop Environment.
The actual command to run is xfwm4. To run it in the background use xfwm4 –daemon. The window manager is responsible for the placement of windows on the screen, provides the window decorations and allows you for instance to move, resize or close them.
xfwm4 adheres strongly to the standards defined on freedesktop.org. Consequently, special features such as making windows borderless, or providing an icon for the application must now be implemented in the application; you can no longer use the window manager to force different behaviour. One of the great features of xfwm4 is its themeability. The window decorations (borders, title bar and window buttons) can be configured by using window manager themes.
xfwm4 offers multihead support, for both xinerama and real multiscreen modes, useful when you have more than one monitor connected to your computer.
xfwm4 can be run stand-alone, but if you use it this way, you will need the Xfce Settings Manager if you want GUI settings management. Tasks other than managing windows, like setting a background image or launching programs, need to be performed by other programs.
xfwm4 includes its own compositing manager, which takes advantage of the new X.org's server extensions. The compositor is like a WM on its own, it manages a stack of all windows, monitor all kinds on X event and reacts accordingly. Having the compositing manager embedded in the window manager also helps keeping the various visual effects in sync with window events. If you want to use the compositor, you have to build xfwm4 using the –enable-compositor configure option. In any case, you can disable the compositor on xfwm4 startup using the '–compositor=off' argument.