root window:
At the top of each hierarchy is a root window,which covers each of the display screen.Each root window is partially or completely covered by child windows.All windows except for root window,have parents.
window note:
1 Each window or pixmap has its own coordinate.
2 X does not guarantee to preserve the contents of windows.Programs must be prepared to regenerate the contents of windows on demand.
3 X also provide off-screen storage of graphic objects,called pixmaps.Single plane (depth 1) pixmaps are sometimes reffered to as bitmaps. Windows and pixmaps together are reffered to as drawable.