Windows have several states of visibility. Already shown is the visible property which shows or hides the window.
Windows can be minimized, which is equivalent to hiding them except that they still appear on the taskbar (or are minimised to the dock, on OS X). The user can minimize a window by clicking the appropriate button in the title bar. You can also programmatically minimize a window using the minimize method (there is also a corresponding maximize method).
When a window is made visible the on_show event is triggered. When it is hidden the on_hide event is triggered. On Windows and Linux these events will only occur when you manually change the visibility of the window or when the window is minimized or restored. On Mac OS X the user can also hide or show the window (affecting visibility) using the Command+H shortcut.