Like previous games, Pokémon Black & White has included a varierty of swarms. These swarms, like the past swarms, activate after you have received the National Dex, change each day at midnight and go through a random selection of possibilities. The focus of these swarms are Pokémon that don't normally exist within the Unova region and as such can only be obtained in the wild using this method
In order to check the swarms, you need to enter a gate between two areas and look at the ticket going along on the back wall. It will mention that there is a swarm of Pokémon going on in that particular route. Go to that route and you'll find the Pokémon.