Mass Effect 3: Leviathan reveals more about the history of the reapers and tasks Shepard with hunting down a mysterious being hidden at the far reaches of the galaxy. The DLC will include new story, new characters, and even new areas on the Citadel. It's also rumored that the DLC may make some changes to the ending in some way. Of course, those are just rumors, so make of it what you will.
Mass Effect 3: Leviathan is priced at $10 or 800 Microsoft Points. Learn more about the DLC on the Mass Effect 3: Leviathan page.
Source: Mass Effect 3 website