One of the most important differences between Pokémon Black Version and Pokémon White Version is the variety of Pokémon that are exclusive to each game. As you tour the Unova region, you will encounter Pokémon that Trainers playing the other version will not find. The clearest example of this difference is the Legendary Dragon-type Pokémon—in Pokémon Black Version, you'll encounter Reshiram, while in Pokémon White Version, you'll come across Zekrom.
Catch and Trade!
Each version of the game has a handful of wild Pokémon that don't appear in the other version. To complete your Pokédex, you'll need to trade for Pokémon that aren't available in your game.