[photon]

They don't really run fully in the background, unless they take advantage of specific limited functionality that Apple has exposed to background apps (play music, track location, etc).

Otherwise there's not much difference between it being in the background and being closed.. if the app developer has done a good job the app will remember what it was doing much better and there is some resource usage, but in theory the phone is supposed to keep track of that and fully close out background apps if necessary.

But if you are like me and obsessive about that stuff, then you can press and hold on any backgrounded app (after bringing up the task switcher), and you can then manually close them.