So, like any new generation, each one revolves around a set of legendary Pokemon that defines the generation (well, sorta for the first generation) such as Lugia and Ho-oh. Black and White is modeled off the Ying Yang theme, thus creating the legends of this generation.
This is Reshiram, the White Yang Pokemon, who is a Fire/Dragon type (about time they came up with this typing combo). It looks somewhat legendary, and I think the typing sort of suck, because now it is weak against Rock and Ground moves, but at least it resist the Ice Moves that many dragons have issues facing.
The counterpart legend is Zekrom, the Black Yang Pokemon, who is both Electric and Dragon type. In my opinion, it looks more Dark/Dragon type, but that we need an Electric/Dragon Pokemon. It's typing does not really give it much defensive advantage, since you don't see a whole lot of Flying or Steel attacks, plus now it is weak against Ground attacks. Of course, it is double resistant to Thunder, and it's Electric attacks will smoke any water types *cough Kyogre* that decides to face it (if you aren't named Swampert or Quagsire).
I hope this legendary Pokemon are somewhat interesting, seeing that Palkia and Dialga were ehh in my opinion.