Dawnguard is an expansion to Skyrim, which itself is an expansion to the Elder Scrolls series of RPGs. Now, Skyrim is a big game all by itself, but what it really doesn’t have is vampires everywhere. Dawnguard fixes that, setting the player up to either hunt down all the nasty bitey types, or giving the option to join their blood-soaked ranks. There’s also something about a crossbow, I don’t know. Check out this review. IGN has the story.
There were vampires in Skyrim before Dawnguard, but they didn’t really seem to do anything beyond being vampires. No spooky powers, no ancient society of the undying, no real reason to become one. Maybe this new expansion will put a little more meat on the dead bones of vampire gameplay.