Android is designed so that users never "close" an app (there's no close button); instead, they switch between apps. The system closes apps in the background when necessary to free memory, but all apps are supposed to save and restore their state so that the user is presented with the illusion that the app never stopped running.
This is untrue. You can close an app manually by pressing the bottom right software button, which shows your foreground apps, and then swiping the app window off the screen.
The state is lost and, for most apps, background processes are killed.
