Angels are celestial, winged beings who serve Heaven. Not much is known about them, except that they rarely fight against demonic forces. The only Angel to appear on the show so far is Orion. Their total number remains a mystery.
There are more classes of angels, the highest and the most powerful being the archangels. From all of the Archangels (their number is assumed to be very small - at most 5), only Lucifer has fallen, making him not only the only fallen Archangel, but the very first fallen angel.
It was implied by Orion that Angels were created some point before the dinosaurs, as he said he wasn't on Earth while they were alive. God asked them to bow down to humanity, but the archangel Lucifer refused. He, along with the angels that supported him, were cast down by God and the archangel Michael, into Hell serving as a prison for Lucifer and all his demons.
Throughout history, the Angel Orion and has caused many natural disasters, to cleanse the world of evil.