You may have noticed a mysteeeeeeerious humanoid face at the end of the brand-new Avengers: Age of Ultron trailer. That ain't no Avenger I ever heard of, you may have thought to yourself. Sure don't look like a Chris to me — Hemsworth or Evans! That's because he's not an Avenger you've seen before in any of the Marvel Cinematic Universe flicks, and not even in any previous Avengers trailers. But his name is Vision (or "the Vision," depending on how formal you wanna get), and he's very important in superhero lore. Here's a basic rundown on him.
Vision is a heroic, sentient robot who has trouble understanding what you fleshy humans call fee-lings. He first popped up in Marvel Comics in 1968, within the pages of Avengers No. 57, written and drawn by Marvel legends Roy Thomas and John Buscema, respectively. (The cover, with Vision looking like he's about 70 feet tall and about to rock a very high-concept prog-rock concert, remains pretty badass to this day.) He was created by Ultron (the villainous, sentient robot who will terrorize cinemas everywhere this May) and sent to destroy the Avengers. Long story short, he gained freedom from his sinister master and eventually became a full-fledged Avenger. He's been a member of the team's various incarnations for a substantial portion of the subsequent decades.
He'll be played in Age of Ultron by Paul Bettany (who has hopefully recovered from the blunt-force trauma of promoting Mortdecai), and although confirmed details are scarce, one can assume he'll share some kind of digital relationship with James Spader's Ultron. Perhaps he'll be created by Ultron, as he was in the comics, but it's highly unlikely that he'll finish out the movie as a bad guy. Perhaps he'll be yet another creation of Tony Stark, albeit one that doesn't go all genocidal on everyone (this would make sense, given that Bettany has also provided the voice of Tony's bodyless robo-butler, J.A.R.V.I.S., in previous Marvel movies). By movie's end, he's likely to fulfill a role he's filled in the comics for a half-century: being the team's resident stoic, logical android. Y'know, like Data on Star Trek: The Next Generation: that sort of thing. In Marvel's comics universe, he's also known for having a very tumultuous and biologically complicated on-again, off-again romance with the Scarlet Witch. (They even had kids at one point, sort of, except they were demon babies, and then Scarlet Witch rewrote reality because they weren't around anymore, and oh God, comics are so complicated.) My guess is that even today's progressive audiences aren't quite ready for an inter-mechanical relationship just yet. But hey, never say never!