Task switching
         The Metro screen doesn't have a taskbar, so you'll soon lose track of the apps you've run. To see what's running, though, just press Alt+Tab. Pressing Win+Tab will switch from one running app to the next, and on a touch screen, swiping repeatedly from the left also cycles through running apps.
You could also simply launch an app again and, if there's another copy running, that one will be displayed.
          If you have a keyboard, repeatedly pressing the Windows key will always toggle you between the Metro screen and the last app.
And if you have a mouse, move its cursor to the far left of the screen (roughly in the centre, vertically) and a thumbnail of the last app you used should appear. Click this to relaunch it.
AltTab