If you're not up on your Thor or Nordic mythology, Odin is the father of Thor and the ruler of Asgard. He's your typical white-bearded god sort, fond of wine, women, and wisdom. He's not immortal (none of the gods of Asgard are), but ages slowly, and possesses amazing powers of strength and wisdom. He famously sacrifices his eye for the foresight to prevent Ragnarok, a sadly futile act, but you can't blame a god for trying.
While a lot of people were hoping that the Brian Blessed would be Odin (and my own money was on the recently signed Stellan Skarsgard), you can't really get much better than Sir Hopkins. He'll add just the right amount of gravitas, but he's an actor who can still have fun in a role. I can't wait to see him go up against Loki, and watching him smack down Thor for his arrogance is going to be pure theater. Cheers, Kenneth Branagh!