When you quit an application, the memory it uses is returned to the system for use by other Windows programs and operating subsystems. You can close it using one of these techniques:
Press Atl+F, then X.
Click the program window's Close box (the X at the upper-right corner of the window), or double-click its Control-menu icon (the icon at the upper-left corner of the window).
If you see two Close boxes, the one closer to the corner of your monitor closes the program. The inner one closes the currently open document, but leaves the application running.
Right-click the program's Taskbar button and choose Close from the shortcut menu.
If you haven't saved the changes you've made to your document, you'll be offered the chance to do so before the program shuts down all the way.