Skyrim Dragonborn is the third official expansion (DLC) for the game The Elder Scrolls V: Skyrim. (The first official expansion was Dawnguard and the second was Hearthfire)
Dragonborn was released by Bethesda on December 4, 2012, exclusively for the Xbox 360 (for $20/1600 Microsoft points), and is scheduled to be released for the PC and PS3 early 2013.
How to start the Skyrim Dragonborn DLC
See: Skyrim Dragonborn: How to Start