Marvel showcased footage from their upcoming Phase 2 movies Thor: The Dark World, Captain America: The Winter Soldier and Guardians of the Galaxy at the studio's 2013 Comic-Con panel. However, it was all buildup to the final reveal: the official title for writer-director Joss Whedon's sequel to The Avengers, which is... wait for it... The Avengers: Age of Ultron.
Whedon took the stage in Hall H at the conclusion of Marvel's Comic-Con panel, in order to introduce a brief teaser for the second Avengers movie, which is also the final installment in Phase 2 of the Marvel Cinematic Universe. The preview included material from the first Avengers film, spliced together with new footage (showing a man approaching an unidentified temple) and the reveal of CGI images that form Ultron's face - followed by the official logo, which is featured as the header image for this article.
[UPDATE: Whedon has revealed that Ultron will get a new origin story in the Avengers sequel.]
Ultron, for those not familiar, is a Marvel villain who was introduced back in the 1960s. The character - originally identified as Ultron-5, the living automaton - is the creation of Henry Pym (a.k.a. Ant-Man), a robot who developed self-awareness and intelligence, but then formed an intense hatred for its "father" and evolved into a supervillain.
The Avengers - in printed form - have fought multiple upgraded versions of the dangerous robot, over the decades since the character's inception in the comic books. As far as his general qualities are concerned: Ultron customarily possesses super-human intelligence and data-processing capabilities, among other "powers" that make him a formidable opponent for Earth's Mightiest Heroes.
Avengers: Age of Ultron, so it seems, will therefore be Hank Pym's introduction to the MCU, before the character gets his own standalone feature in 2015 with Ant-Man: the first installment in Marvel's Phase 3. Whedon using the character in his Avengers sequel script would also set the stage for the filmmaker to include Janet van Dyne/Wasp in the film, given her connections to Pym (she previously came close to making the cut in the first Avengers movie).
Similarly, assuming that Ultron's origins are not significantly changed or altered for Whedon's film, this means that Avengers: Age of Ultron should be the third Earth-based adventure in Phase 2, after Iron Man 3 and Captain America: The Winter Soldier. That appears to be the case, given the previously-established importance of the Captain America sequel in the overarching story in Phase 2 (according to the previous reports on that subject) - and thus, the expectation is that Cap's new adventure will place down the immediate groundwork for an Avengers reunion.
Iron Man 3 releases on DVD and Blu-ray on September 24th, 2013.
Meanwhile, Thor: The Dark World opens in theaters on November 8th, 2013, followed by Captain America: The Winter Soldier on April 4th, 2014, Guardians of the Galaxy on August 1st, 2014, The Avengers: Age of Ultron on May 1st, 2015, Ant-Man on November 6th, 2015, and unannounced films for May 6th, 2016, July 8th, 2016 and May 5th, 2017.