For skyrim, there have been 3 DLC's released, Dawnguard, Hearthfire, and then Dragonborn, in that order. Each DLC adds new, items, physics, and story and purpose to the game.
Dawnguard is the first of the DLC's. This add's a whole new story to the game. There is a guild known as dawnguard, who fight vampires. There is also the vampires, which include a new type known as a Vampire lord, similar to a gargoyle than a vampire. You can either be a dawnguard, or a vampire lord (if you are already a werewolf, this is when you can remove that to be a vampire). This also adds several new enemies, followers, such as armored troll minions, and weapons, the most noticable being the Crossbow. However, this is very buggy on Steam and crashes constantly in certian areas.
-Written by Dovakiin J9j9