Try this analogy and see if it helps. Imagine that you're sitting in a room with a bunch of your friends (your apps.) From time to time you speak to one of them and they respond. Otherwise, they sit quietly.
Perhaps one of your friends is interested in the news so he might go get the paper once in awhile, but otherwise he just sits in his chair. Another friend might look out the window from time to time to see if it's raining. Otherwise, he, too, just sits there. And one of your friends is in charge of the music so he waits patiently for you to tell him to put on a cd.
You don't have to tell any of your friends to leave the room when you aren't talking to them unless the room gets so crowded that there's no place to sit. Then you have to politely tell one of your friends to step outside for a little while. You might choose the fattest friend or you might choose the guy you haven't spoken to very often. Whomever you choose to send outside the room will wait patiently outside the door and come back in when you need him.
If you get tired of the guy running to door to get the newspaper or the guy checking the mail, you can (usually) tell them to stop doing it so often.
You don't have to shoot any of your friends (uninstall them) or banish them from the room (kill the apps) when you aren't talking to them unless they're drunk and keep shouting at you while you're trying to talk to someone else. And fortunately, those people are pretty rare.
That's a non-technical description of your operating system. Let it do its job.
