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 a calendar, you’ll save time in the long run by keeping it open all day long. (You can always minimize its window 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 spot (at the upper-left corner of the window).
Right-click the program’s taskbar button; from the shortcut menu, choose Close or Close Group.
Point to the program’s taskbar button; when the thumbnail preview pops up, click the little button in its upper-right corner. (If the program had only one window open, then the program exits.)
Press Alt+F4 to close the window you’re in. (If it’s a program that disappears entirely when its last document window closes, then you’re home.)
Press Alt+F and then X.
After offering you a chance to save any changes you’ve made to your document, the program’s windows, menus, and toolbars disappear, and you “fall down a layer” into the window that was behind it.