Angels are long-lived, powerful beings that inhabit Heaven and serve God.
Angels have wings, and demons describe them as bright, having light.
A human can become an angel in much the same way as a demon, by making a contract with an angel (demons say they then become puppets of Heaven). It was also revealed that pagan Gods can become angels, as was the case with Baalberith, before he fell. 
Demons describe angels as being far more vicious than them.
Angels have to repeatedly Sleep for a long time on order to keep their long life and replenish their power.
Jeanne d'Arc is strong enough to be given the title archangel.
Angels of PresenceEdit
Noble angels are higher ranking angels. Azrael is one of the noble angels, and Chief of the Cherubim.
Angels of OriginEdit
Angels that were born as angels, and weren't human (or other) before.
Humans that were turned into angels. It's said that when one becomes an angel, they lose their human personality and become a pawn of Heaven. Upon death, they are given new bodies by God, Michael called Metatron, a formerly human Angel, a clay doll.
Angels that used to Gods of their own Worlds but were given ranks as Angels ("summoned and canonised, given status") and accepted into Heaven (as their Worlds were either destroyed or conquered by Heaven).