This game has an important requirement, however: you need to have the ROMs for the games in question available on your smartphone. For the game to work, you absolutely must have the ROM for Pokémon Black/White, and you can get additional content for the same game if you also have FireRed/Emerald, plus better graphics with HeartGold/Soulsilver.
Once you register (which you can do for free), you can access the servers and start playing. Customize your avatar and select your main Pokémon, then leave your village to start catching all the Pokémon in the Pokédex.
As for gameplay, it's typical for Nintendo DS Pokémon games. You move your character with a virtual D-pad and interact with your environment in real time, while combats are fought by turns as in the rest of the games from this saga.