(WARNING!!! Spoilers Follow)
Michael, portrayed by Christian Keyes and others including Matt Cohen (possessing John Winchester), Jake Abel (possessing Adam Milligan), and Jensen Ackles (possessing Dean Winchester), is the first archangel loyal to his father God and the plan to give the Earth a clean slate... even at the cost of every human life.
Michael is first mentioned in the first few seasons of the show and is wanting Dean as his vessel for a battle with Lucifer, which will begin the apocalypse. He first appears to Dean, trying to convince him to give his permission to be his vessel by taking the form of another Winchester, a young John. He reveals that anyone in the Winchester blood line can be his vessel.
However, he does not possess Dean, but instead possesses Adam, Sam & Dean's half brother. The planned events go awry when Sam is able to take control over Lucifer and brings them both into the cage in hell. Both Sam and Lucifer eventually escape from the cage, while Michael and Adam remain.
Another version of Michael surfaces in "Apocalypse World" where his mission has changed that world into a desolate place for humans. He eventually crosses to "our world" and Dean agrees to be his vessel so they can defeat Lucifer, however, Michael does not leave Dean and takes control over him to "cleanse" this world too. And thus, the battle for Dean begins again...