Make a window or application inactive. Not to be confused with minimizing (Windows), which hides a window without rendering it inactive.
To close an application, select "Exit" or "Quit" from the File menu. To close a window, click the "X" button in the top right corner (Windows 95 and higher) or the square button in the top left corner (Mac). (NOTE: In Windows, closing an application's "master window" will close the app itself.)