user3416: I want to get a strong engine to practice while I'm not home. Look at: http://stockfishchess.org/download, we see a list of apps to download for the Stockfish engine.
Why are there three apps to downloads for the same engine? Which one should I choose? What are their advantages and disadvantages? Which one is the strongest? Which one is the easiest to use? What functionality do they offer?
PS: I have an iPhone 5 and Android tablet.

user3420: You see those distributions because they're built for each platform. If you have iOS, use the iOS binary. If you have Android, use the Android binary.
Now, you ask for which one is strongest, advantages etc. On theory, Droidfish on Android is the weakest among three apps (SmallFish, Stockfish and Droidfish). It's weakest because it's a 32 bits app while the other two are 64 bits.
SmallFish and Stockfish are the two choices that you can make on the iOS (Android has only one). While the Stockfish app is older, SmallFish has better functionality includes evaluation graph, annotations and Online Store. The Stockfish app is solid but it offers nothing more than regular game playing. SmallFish goes for more.

user2429: On the Android tablet, you download the version for Android, and for the iPhone you can download the iPhone version. 
Stockfish engine's source code is written in C++, and therefore it has to be compiled for the platform (Windows, Android, iPhone, etc.) on which it will run. Thus, you have different "versions" (not in the strict sense) of the same engine.  
For Android, I recommend you download the app called DroidFish. I use it myself. It is very convenient. 

3416: <p>I want to get a strong engine to practice while I'm not home. Look at: <a href="http://stockfishchess.org/download" rel="nofollow">http://stockfishchess.org/download</a>, we see a list of apps to download for the Stockfish engine.</p>

<p>Why are there three apps to downloads for the same engine? Which one should I choose? What are their advantages and disadvantages? Which one is the strongest? Which one is the easiest to use? What functionality do they offer?</p>

<p>PS: I have an iPhone 5 and Android tablet.</p>
 3420: <p>You see those distributions because they're built for each platform. If you have iOS, use the iOS binary. If you have Android, use the Android binary.</p>

<p>Now, you ask for which one is strongest, advantages etc. On theory, Droidfish on Android is the weakest among three apps (SmallFish, Stockfish and Droidfish). It's weakest because it's a 32 bits app while the other two are 64 bits.</p>

<p>SmallFish and Stockfish are the two choices that you can make on the iOS (Android has only one). While the Stockfish app is older, SmallFish has better functionality includes evaluation graph, annotations and Online Store. The Stockfish app is solid but it offers nothing more than regular game playing. SmallFish goes for more.</p>
 2429: <p>On the Android tablet, you download the version for Android, and for the iPhone you can download the iPhone version. </p>

<p>Stockfish engine's <a href="https://github.com/official-stockfish/Stockfish/tree/master/src" rel="nofollow">source code</a> is written in C++, and therefore <strong>it has to be compiled for the platform</strong> (Windows, Android, iPhone, etc.) on which it will run. Thus, you have different "versions" (not in the strict sense) of the same engine.  </p>

<p>For Android, I recommend you download the app called <a href="https://play.google.com/store/apps/details?id=org.petero.droidfish&amp;hl=en" rel="nofollow">DroidFish</a>. I use it myself. It is very convenient. </p>
