One of the most common Christian theories in Angelology is the idea of the angelic hierarchy, made famous by the work of Dionysius the Areopagite in the 4th Century AD. This definite hierarchy contains three concentric circles with God at the center. There are three distinct “Choirs” of Angels in each of the concentric circles.
The outermost circle is the closest to the physical world, interacting most often with man.
Angels- These beings are the lowest rank on the celestial ladder. Think of them as “Guardian Angels”. The Angels serve as foot soldiers in the Army of Heaven.
Archangels- These beings are the next step up from Angels. They follow the affairs of lesser angels and groups of men (clans, villages, etc...) The Archangels serve as the leaders in the Army of Heaven.
Principalities- These are the “Angels of Nations” they have dominion over the Archangels and Large groups of men (nations, tribes, etc..) The Principalities serve as beaurocrats and generals in the Army of Heaven.
The second circle in are more distant and removed from the majority of humanity.
Powers- These angels serve as the celestial “Border Guard” between heaven and Hell. They have power over demons and sometimes are controlled by them. The majority of “Fallen” angels seems to come from this group due to their treacherous duties on the borderlands. Their duties include ensuring that the souls of men reach their appropriate reward (Heaven or Hell) and serving as the main force of defense in case of a demonic attack of heaven.
Virtues- These angels serve God by providing the mechanism for miracles. They actually control the “Song of Life” of the world and dole out the required blessings/punishments demanded by the divinity.
Dominions (Dominations)- These angels serve God by monitoring the Powers and Virtues. They maintain a “cosmic” balance that reaches beyond the earth to the entire physical plane.
The innermost circle of Angels are the closest to God and server him directly. They do NOT necessarily look like winged humans.
Thrones (Ophanim)- Their primary duty is as the “Chariot” or “Throne” of God. These angels usually are described as looking like wheels. They also are considered to be dispensers of God’s Justice.
Cherubim- These angels, despite common belief, are NOT viewed as portly children with undersized wings, but rather usually as Assyrian Sphinxes in form. In many belief systems Lucifer was of their number. These Angels are “Spirits of the Harmonies. The guardians of the fixed stars, keepers of celestial records, bestowals of knowledge.”
Seraphim- The uppermost section of the hierarchy, these are the angels closest to the throne of God. It is they who unceasingly chant the Trisagion, "Holy, holy, holy..." and encircle the throne, existing off the love emanated by God. It is said that the light they give off is so intense, that not even other divine beings may look upon them. There is said to be four of these angelic beings. They are listed as the four holy beasts in the book of Revelation and are also described as angels with four faces and six wings.