When you exit, or quit, an application, the memory it was using is returned to the Windows pot for use by other programs.
If you use a particular program several times a day, like a word processor or calendar program, you’ll save time in the long run by keeping it open all day long. (You can always minimize its window to get it out of the way when you’re not using it.)
But if you’re done using a program for the day, exit it, especially if it’s a memory-hungry one like, say, Photoshop. Do so using one of these techniques:
Click the program window’s Close box, or double-click its Control-menu icon (at the upper-left corner of the window).
Right-click the program’s taskbar button and choose Close from the shortcut menu.
Press Alt+F4 to close the window you’re in. (If it’s a program that disappears entirely when its last document window closes, you’re home.)
Press Alt+F, then X.
If you haven’t yet saved the changes you’ve made to your document, the program offers the chance to do so before it shuts down all the way. Finally, after this step, program’s windows, menus, and toolbars disappear, and you fall “down a layer” into the window that was behind it.