Android, MOST of the APPS stay active, as there is no such thing as IDLE apps.
This is totally incorrect, although it seems to be a common misconception.
Just as many people get confused by the iOS recently-used apps strip and mistakenly think those apps are all still active, many people also see apps in Android task managers and mistakenly think all those are active.
In actuality, Apple added
almost exactly the same model of "freezing" apps when they're pushed into the background, as Android is already using.
In both OSes, most apps are paused in place, doing nothing, ready to be started again as-is. In both cases, the least-used apps are removed if memory is needed. In both cases, the apps that are about to be removed can implement a special method to store state.
The biggest difference from Android is that most iOS apps have not been rewritten or recompiled to take advantage of this fast-app-switching model.
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As for actually doing stuff in the background, both OSes allow apps to do at least the most common and useful operations: transferring files, playing music, waiting for timed events, and watching for location changes.
Android allows a few more background operations if an app is written for it (usually if they're a widget meant to do so). No doubt iOS will eventually add these things as well... just the same as they have already added some to the original iOS.
There seems to be a common and naive myth that allowing background tasks is somehow bad. Does that mean iOS 4 is evil compared to previous versions? Of course not. Devices advance, and they become far more useful when they can do more for us without being asked.