The story of Dawnguard involves the return of a Volkihar vampire lord named Harkon, who seeks to end the tyranny of the sun. Harkon intends to use the power in the ancient Elder Scrolls to accomplish his goal, while an order of vampire hunters known as the Dawnguard seek to stop him.
The Dragonborn is given the option either join the Dawnguard and help defeat Harkon or join forces with the Volkihar vampires.
Dawnguard can be started by listening for rumors or asking Hold Guards about a group of vampire hunters who are recruiting. This will steer the Dragonborn in the direction of Fort Dawnguard and the add-on will begin.
For detailed information about the release of the add-on, see the full page.
The two main factions central to the add-on are the Dawnguard, an order of vampire hunters dedicated to eradicating Vampires, and the Volkihar Clan a shadowy group of Vampires who seek to blot out the sun.