A window is a rectangular area of the screen, with a border and a title bar at the top. You can think of a window as a screen within the screen. Each window displays an application, enabling you to have more than one application visible, and work on more than one task at a time.
You can control a window's position on the screen, as well as its size. You can control which window overlaps other windows, so the one you want to work with is completely visible. For more information about moving and resizing windows, see Working With Windows.