Basically Cho has this machine which creates synthetic tissue. Ultron controls her using the mind gem to create a body she infuses it with vibranium then Ultron puts in the mind stone. He starts to upload but Wanda reads Ultrons base conscious through 'Vision' which she can't do with Ultron normally, she sees Ultron is planning to wipe out humanity. The Twins leave. On the truck Ultron continues to upload but Avengers attack. So some of Ultron base stuff is in 'Vision's' body. The Avengers capture it and take it to Stark. Who then decides to upload Jarvis. Cap and Tony fight over this, QS pulls the plug on Vision. Mini Avengers can't of battle. Thor arrives finishes booting up Vision. Vision steps out kind of crouches for 5 seconds looks at Thor jumps at him. Thor tosses him across the room. Vision suddenly stops looks outside for 5 seconds then turns around and starts speaking. Couple of minutes of talk where Vision say he is neither Ultron or Jarvis then they go off to fight Ultron.
So you see we really know nothing about how he truly came to be or how he works, Cho seems to have a machine that can create a synthetic body which can fly without propulsion and phase through objects.
We don't really know if Vision is a new AI or if he is a mix of Ultron and Jarvis. We don't really understand what the mind stone is really about, it appears to give Ultron consciousness, Jarvis says he detects code inside the stone so does it give Jarvis conscious or does the mix of Ultron give him conscious.
But he is still pretty awesome and the best new character imo.