A Gdk.Window is a rectangular region on the screen. It's a low-level object, used to implement high-level objects such as Gtk.Widget and Gtk.Window on the GTK+ level.

A Gtk.Window is a toplevel window, the thing a user might think of as a "window" with a titlebar and so on; a Gtk.Widget may contain many Gdk.Windows. For example, each Gtk.Button has a Gdk.Window associated with it.