Ultron is a superintelligent artificial being who has one motto: "Death to humanity".
And he always comes back. Usually deadlier than before.
Nobody knows why he has such a beef with humanity. He just does. I guess something about us annoys him.
Make no mistake, though. He is in the top five of most dangerous enemies of the Avengers - and the others are the likes of Kang the Conqueror, Loki of Asgard, Doctor Doom and Thanos. He once exterminated the population of an entire country... in an afternoon. And when the Avengers and the UN invaded, he sent the cybernetically animated corpses of the people to stop them. He took on all the Avengers at once, defeating all of them (including Thor) and only going down when Hank Pym layed the smackdown with Antarctic Vibranium (a unique material which dissolves all metals on contact).
In the comics, Ultron was originally created by dr. Henry Pym, one of the founding Avengers. But because they told the story differently in he movies, I suspect Ultron will instead be an evolved artificial intelligence created by Tony Stark.
After all, Tony intends to reduce his activity as Iron Man for Pepper's sake (and Robert Downey jr's sake, I imagine); but he can't leave the Avengers high and dry. Now, I guess he could ask War Machine/ Iron Patriot, but James Rhodes (again, unlike in the comics) is a USAF officer, and that would link the Avengers directly to the US government and create a conflict of interest for Rhodey.
So what does he do? Well, he already has created several artificial intelligences - think of Dummy and Butterfingers, his lab assistent robots, and JARVIS, the AI who runs his household and regulates his armor systems. Why not create an artificial Iron Man?
That might explain why Ultron is apparently kind of ambiguous - not so purely evil and determined as in the comics (Honestly, Ultron is very centered. It's "kill humanity" 24/7 with that guy. Well, that and outshining his creator). He feels demeaned by the attitude of the other Avengers, who are not comfortable having an artificial entity on the team, and tries to prove he is better than any of them... and then better than all of them. He wants to prove himself. Perhaps he even creates the Vision to prove he is better than his own creator.
At least that is what I think. The teaser, after all, involved an Iron Man helmet becoming Ultron's head.