In Medieval Christianity, angels are divided up into three hierarchies, each made up of three orders:

Seraphim - the highest order of angels, depicted with six wings (or sometimes with just a head and wings, like the cherubim). In the Bible, only the seraphim and the cherubim are depicted with wings. They are sometimes wearing red and surrounded by fire.
Cherubim - gatekeepers of Eden, with eyes on their wings. Sometimes they stand on wheels or are carrying disks (from the vision in Ezekiel 10)
Thrones - seated on a throne or standing next to one


Dominions - carry a scepter or an orb as a symbol of their authority
Virtues - their imagery is very irregular, since "the working of wonders does not lend itself to a distinct iconography"
Powers - usually holding swords, since they are continuously fighting demons and forces of evil


Principalities - in armor, carrying weapons
Archangels - unless they are Michael, Gabriel, or Raphael, they carry a trumpet as a sign of revelation
Angels - closest to humanity, no distinctive appearance