In its earliest days and versions, opening a whole bunch of apps in a row on an Android device would leave many of them in that device's memory, causing your short attention span to gum up the works.
Memory usage improved, though, and popular "task killers" became an unnecessary utility. Then, in Android 4.0, there appeared a list of recent applications, which seemed meant for easy app-switching. And yet: you could swipe apps away. What did that do? Well, it seemed to "close" them, because they'd load up fresh, from their launch screen, the next time you tapped on them. But then again, if it was something like an email app, or Twitter, you'd keep getting notifications. So what does "swiping" do, exactly?
As is often the case: ask an experienced developer, one who can write in Plain English. Dianne Hackborn works on Android at Google, and uses Google+ to spark a number of discussions on the platform and its intricacies. Here's what Dianne says about "swiping" an app, per a Android StackExchange discussion, via a Lifehacker post on that topic:
Actually, removing an entry in recent tasks will kill any background processes that exist for the process. It won't directly causes services to stop, however there is an API for them to find out the task was removed to decide if they want this to mean they should stop. This is so that removing say the recent task of an e-mail app won't cause it to stop checking for e-mail.
So if, say, Facebook is being wonky at uploading a picture and you want it to stop trying, you can swipe it out of the recent apps list and it will stop that action. It's the same as if you went into Facebook and tapped the "Back" button until everything went away. But Facebook will still be able to ping you about messages, event invitations, and all those things that people are doing with their lives that are so much more fulfilling than whatever it is you are doing at that moment.
So what if you want to actually triple-kill-and-bury an app, one you believe is sucking up battery life or killing your data connection or just generally being bad? Do this, Hackborn writes:
... (L)ong press on recent tasks to go to app info, and hit force stop there. For stop is a complete kill of the app -- all processes are killed, all services stopped, all notifications removed, all alarms removed, etc. The app is not allowed to launch again until explicitly requested.
Sometimes the Android answers are just falling from the sky. Into StackExchange. Have a great weekend.