Assassin's Creed: Embers is an animated short film that will be released with Assassin's Creed: Revelations Animus, Collector, and Signature Editions. It will also be available on XBL and PSN on November 15, the same day as AC: Revelations. Even though you can't ineract with Embers, it was made with the same graphics, characters and style as the game.
After Assassin's Creed: Revelations, Ezio goes on to live a normal life. Well, as normal a life a master assassin can have. In Assassin's Creed: Embers Ezio has a family, complete with a wife and two children. As the trailer unfolds you see that his "simple" life is plagued with threats of being attacked.
I attended the Assassin's Creed panel at Comic-Con and the creators gave these words of advice for hardcore AC fans: Don't watch Embers until you beat AC: Revelations, that way you won't have anything spoiled. (But I guess that fact that Embers exists is a spoiler that Ezio lives through Revelations.)