Each program uses "memory" (the amount of RAM memory your computer has) in your computer.  To switch from one program to another, you can mouse click on the program you want to use located on the Taskbar at the bottom of the screen.

OR

Use the ALT-TAB key:
1.  Press and hold the Alt key.
2.  Press the Tab key until the program you want to use is marked.  Release the buttons.