wm overrideredirect window ?boolean?
If boolean is specified, it must have a proper boolean form and the override-redirect flag for window is set to that value. If boolean is not specified then 1 or 0 is returned to indicate whether or not the override-redirect flag is currently set for window. Setting the override-redirect flag for a window causes it to be ignored by the window manager; among other things, this means that the window will not be reparented from the root window into a decorative frame and the user will not be able to manipulate the window using the normal window manager mechanisms.
[Explain idea.]
[Explain uses--balloon help, screensaver, various wm manipulations, much else.]
See also:
[Compare window zooming.] [Is that name as bad as CL thinks?]
FW: See also TkTrans, which allows for borderless windows that are still fully managed by the window manager.