You can close them actually, but they basically hibernate so you can switch back to them quickly. If you swipe from the left (or put your mouse in the upper lef

You can close them actually, but they basically hibernate so you can switch back to them quickly. If you swipe from the left (or put your mouse in the upper left corner) it will show a list of all the apps and you can pick one. Or just swipe a couple of times to swap between them all. Also you can pull one out and dock it to the left or right so you can have two apps running simultaneously. You can right click or press and hold on the apps in the left side list and a menu with the option to close them appears. And actually if you pull the app off of the left and off the bottom edge it will close it also. There are gestures and keyboard shortcuts for all of this. But yes, the apps will actually be hibernated if they're in the background long enough and they will also be closed completely if not switched to eventually. There are api's for the apps to detect these switches and persist state. And the process will be forcefully killed if the app takes to long doing it.