basically everything. you can scroll back to a not infinite number, but if I were to go all the way back it'd be something I haven't looked at in a long time. That does not mean the app is running though. If you haven't opened an app in a while, the OS will free the memory it was using when another app needs memory.
Don't think of it as a task manager, because that's not really what it is. It's called the multitasking view, and in reality it is is just a list of things you have opened in chronological order. It has little bearing on whether an app is active or not.