An is the installation file for any Android app. Think of it as a ".exe" file for Windows. It contains the core code and basic assets of a game. For modern games, especially first-person shooters, the basic APK is often quite small because the bulk of the data—graphics, sounds, and maps—is stored separately.
Most "Black Ops 2" APKs found online are either fan-made companion apps containing only maps or information, or Zombies references rather than the full game.