It's usually easy to see to how to get to Win 8 app. Once under Metro UI or an app, is there way to get back to Classic Shell without re-logging in.
You want to go back to the desktop? Simply select the Desktop tile in the start screen or press Win+D.
Great! If I want to be stupid, how do you get from Classic Shell to Metro UI (but not an app).
By default Shift+Win or Shift+click. You can change it from the Controls tab in the settings.