From ComicVine: Set in an apocalyptic future, Ultron has returned to earth, and ruined it, using his superior technology to enslave the mankind. Certain villains do as Ultron commands and get higher privileges but at the end of the day Ultron rules. Most of the heroes live underground whilst thinking of a way to rid the world of Ultron. Amongst these heroes are Captain America, Luke Cage, Iron Man, and Emma Frost. Hawkeye is with them, but he's willing to go out into Ultron's world if it means saving his friends. The event follows the heroes plan to prevent Ultron from ever returning from the future, and the consequences of these actions.
That's the plot from the epic Marvel Comic book event, the most interesting point, movie wise, being the inclusion of X-Men characters. Of course they could write them out of it entirely...but...what if.... The real question is, with Days of Future past taking on the "time travel for the sake of preventing an apocalypse" theme, how will Joss Whedon pull this story off? Ohhh...many..many...many ways. The mind races with possibilities.
Update: Empire Magazine says "The title is 'borrowed' from the comic, but bears no relation to that arc." ...so I'd just expect a Terminator-esque story line! Terminators vs Avengers. I'm in.