:input-output windows are the standard kind of window you might expect. Such windows receive events and are also displayed on screen. :input-only windows are invisible. They are usually placed above other windows in order to trap or filter the events. You cannot draw on :input-only windows.

(define-g-enum "GdkWindowWindowClass" gdk-window-window-class
  (:export t
   :type-initializer "gdk_window_window_class_get_type")
  (:input-output 0)
  (:input-only 1))  

:input-output

Window for graphics and events.

:input-only

Window for events only.