You just drag your mouse over to the left hand side of the screen. I mean all the way over..then you'll see a little preview window of the last window you were viewing.
You can click and hold and drag to the right and let go of it to bring it into focus.
Or just drag your mouse cursor all the way to the very bottom left hand corner of the screen to bring that will bring up the start menu. You can just click in the vey corner and it will bring the main Win 8 window back up into focus.
Or just press the *Windows key* on your keyboard.
Or press Windows Key + D to go directly to the desktop.
Or Windows Key + D + Tab
or Windows Key + Tab to cycle through.
or CTRL + Windows Key + Tab