In Windows 8, you can close an app by moving the mouse to the top of the screen until the title bar appears, and then click the X button. But how do you do it if you are on a touch-only device?
The way to close an app using only touch is to drag from the top of the screen to the bottom. If the app is not currently on screen, you can pull in the switcher from the left edge, and then drag the app from the switcher to the bottom of the screen.
There are some variations on this gesture, depending on how you want to close the app.
If you simply drag the app to the bottom of the screen, then the app is closed, which means that the window disappears from the screen, and the app is suspended. The app is not removed from memory immediately, however. Only if there is a need for the memory will the memory for the closed app be reclaimed. The system hangs onto the memory in case you relaunch the app, in which case it doesn't need to fire up the entire app from scratch; it can just resume the suspended version of the app.
If you want to close the app and terminate it, then drag it to the bottom of the screen, and then hold it there. After a few seconds, the picture will flip from a thumbnail of the app to its splash screen. Now let go. This closes the window and terminates it.
A variation on the above gesture is to drag the window to the bottom of the screen, hold it there until it flips to the splash screen, and then move it up to the top half of the screen, and then let go. This is the combo close-terminate-restart gesture: It closes the app, terminates it, and then immediately relaunches it.
These gestures also work with the mouse. You can grab the window from the switcher, or you can move the mouse to the top of the screen and grab its title bar. The rest of the gesture remains the same.
Windows 10 in tablet mode retains the core drag-to-bottom-of-screen gesture, but it dispensed with the variations. Dragging an app to the bottom of the screen closes and terminates it.
Bonus chatter: These gestures are admittedly difficult to discover, but gestures in general are difficult to discover. After all, there's nothing obvious that tells you that a five-finger-pinch closes an app on an iPad. I'm told that Apple advertisements for the iPad actually served two purposes. One was, of course, to sell the iPad. But the other purpose was to serve as 30-second training videos on how to use an iPad. Windows ads¹ included the top-to-bottom-drag close gesture, but I guess people just tune out Windows ads and don't realize that there's a training video hiding inside them.
¹ Okay, maybe not this one.