Assassin's Creed: Pirates is a mobile game not to be confused with the free companion app that will be tied in with Assassin's Creed IV: Black Flag. This is an actual game that has no direct tie-in with the console/PC versions.
You play as Alonzo Batilla during the Golden Age of piracy in the Caribbean. Players can upgrade their ship and engage in huge naval battles all in an effort to claim La Buse's legendary treasure. Essentially it's like AC IV's pirate ship battles, you can even upgrade your ship and expand your crew the same way. It's just the whole game is based on your ship, and there's no "present day" sections either. Most impressive is the game's engine, as the graphics looks pretty great for mobile.
No platforms were stated other than "high-end smartphones and tablets," but you can bet this will be coming out for iOS platforms at the very least. Expect this one sometime in the Fall.