First Appearance: Strange Tales #110 (July 1963).
Appearances: Many, many, many appearances.
Years Active: ?-present.
Dr. Stephen Strange was an arrogant surgeon who was involved in a car accident. Strange recovered from the accident but lost the fine motor control he needed to operate. He despaired and finally hit rock bottom before hearing rumors about an Asian wizard who could heal his hands. Strange made his way to Tibet, only to meet the Ancient One.
After an awkward beginning Strange learned humility and was taught sorcery by the Ancient One. Strange eventually became the Sorcerer Supreme for the Earthly dimension (or only Earth itself, depending on which story you read).
All this is standard and well-known (at least, to readers). What's not so well-known is when, exactly, Stephen Strange began using his sorcery for good. He definitely began his career as a hero in the years before the events of Fantastic Four #1. We know, for example, that in his debut he was already spoken of "in whispers," and Strange is shown speaking with the Watcher at the end of Marvel: The Lost Generation #12. But more than that we don't know.