The swm window manager introduced the virtual root window. From the point of view of the X protocol, this is a regular window. However, X window managers using the virtual root window reparent all windows to it instead to the real root window. Virtual root windows are mainly used by virtual window managers, for example for realizing a number of virtual desktops the user can switch between.
Other clients (beside the window manager) may need to use the virtual root window. The identifier of this window can be found in a window property named __SWM_VROOT in a direct child of the real root window. Technically, many programs use the macros for finding the root windows as redefined by an include file named vroot.h.
The freedesktop window manager specification requires a property named _NET_VIRTUAL_ROOTS of the real root window to contain a list of the windows that are acting as virtual root windows; the virtual root window that is currently active can be found in the property _NET_CURRENT_DESKTOP of the real root window.
The virtual root window is used by the window managers swm, tvtwm, amiwm, and enlightenment. Other systems such as KDE use a virtual background window (which contains the icons, for example) but do not reparent the other windows to this one. The virtual root window is also used by XScreenSaver: when the screensaver is activated, this program creates a virtual root window, places it at the top of all other windows, and calls one of its hacks (modules), which finds the virtual root window and draws in it.
Read more about this topic: Root Window
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