As far as I know, Android apps can add core functionality to the phone. For example, if I download the Facebook app, I can now share my photos from the Camera app via Facebook, without having to actually open the Facebook app itself. Same with Dropbox, Twitter, and the like.
I'm starting to feel this is what is driving the Android fragmentation argument with some iOS users.
iOS's design is much more dependent on Apple implementing core features and interactions (I suspect this is partly by design). The benefit of this is that when Apple does implement them, they are incredibly well done and have that glossy Apple finish that Apple does best.
Android's design allows heavy integration—or sometimes outright replacement of—all sorts of core OS features by third party developers.
Case in point: Google Maps on Android is just a regular app. It's not built into Android (though it does ship by default on basically all Android phones). It receives updates regularly from the Play Store outside of Android's infrequent OS updates. Just being a regular app, however, doesn't prevent it from connecting itself into all sorts of actions throughout Android. You click on an address in a third party and it'll open in Google Maps (or any other third party app that registers itself to handle the "map an address" intent). You speak "Navigate to Starbucks" into Voice Actions and Google Maps will open.
If you want to use a different browser on Android (Chrome for instance) you just install it and make it your default browser (which is an option you get the first time you try to open a link after you have installed it). Want to open reddit links in a reddit specific app? After installing your favorite you'll get the option to always open reddit links in that app (apps can register to handle specific url patterns).
People talk about how you can Share using third party apps on Android but that's really just the tip of the iceberg that the Android Intents system allows. The Intents aren't even just a limited set defined by Android itself. You can create new Intents that the Android developers never envisioned and now your app seamlessly integrates with any number of other applications making use of those Intents. It's a beautiful system.
So while iOS users sometimes have to rely on Apple to add functionality (like Twitter or Facebook), Android users often don't require anything but the third party developer to implement it. I believe this is why fragmentation seems to be a lot bigger of issue to some vocal iOS users than it is to many actual Android users. That's not to say that it isn't an issue, just that it isn't as big of an issue as they might expect from their experience with iOS.
