You can check which Windows 8 apps are running and switch between them by bringing up a hidden bar on the left side of the screen. Note that this bar will not appear unless you have at least one app running.
On a touchscreen; simply swipe to the right from the left edge of the screen. With a keyboard; hold down the Windows key and press Tab. Using a mouse; place the cursor in the top right corner and then move it down the left edge once an image appears.