Dawnguard adds two new questlines revolving around vampires; one where you side with the vampires, and one where you hunt them. It adds a new vampire lord transformation, with it's own perk tree. Werewolves also get a perk tree now, too.
There are a few new sidequests. Also, crossbows. The DLC doesn't add an entirely new map to explore, but there are new locations to visit in Skyrim.
Sometimes I edit my sig so it continues whatever I was posting about.