Windows 8 has a number of system-wide navigational features that are available wherever you are -- on the Start screen, the Desktop, inside a new Windows 8 app or in a traditional Desktop app. The Charms bar is one of them, and the old Windows standby Alt-Tab is another.
Press the Alt and Tab keys on your keyboard simultaneously, and a strip of thumbnails of your running programs appears. While holding down the Alt key, keep pressing the Tab key until you come to the thumbnail of the program you want to run. Release the keys, and you'll switch to that program.