This really should go in the Pokemon questions thread.
But since you're here, yes, there are 13 new Legendaries:
494: Victini - Fire/Psychic
638: Cobalon - Steel/Fighting
639: Terrakion - Rock/Fighting
640: Viridion - Grass/Fighting
641: Tornelos - Flying
642: Voltolos - Electric/Flying
643: Reshiram - Dragon/Fire
644: Zekrom - Dragon/Electric
645: Landlos - Ground/Flying
646: Kyurem - Dragon/Ice
647: Keldeo - Water/Fighting
648: Meloetta - Normal/Psychic and Normal/Fighting
649: Genesect - Bug/Steel
Victini is an event Pokemon, which is obtained by downloaded a special ticket over WiFi.
Reshiram is the white-colored Dragon/Fire legendary available only in Pokemon Black.
Zekrom is the black-colored Dragon/Electric legendary availble only in Pokemon White.
Tornelos is in Black, and Voltolos is in White.
Landlos completes the Flying trio.
Kyurem is the third legendary to Reshiram and Zekrom.
Keldeo is the master of the Fighting trio. (Cobalon, Terrakion, and Viridion)
Meloetta has two formes, the Voice Forme is Normal/Psychic and the Step Forme is Normal/Fighting.
Genesect has a move called Techno Buster. You can attach "cassettes" to him and change the move's type to either Ice, Fire, Water, or Electric.
The last three are event Pokemon that have not yet been officially revealed.
So that brings the total number of Legendary Pokemon to 48.
In total, there are 156 new Pokemon in Black/White, bringing the grand total to 649.
You can see all the new Pokemon
here. Click on their name to go to their Pokedex page.