According to medieval Christian theologians, the Angels are organized into several orders, or Angelic Choirs. The most influential of these classifications was that put forward by Pseudo-Dionysius the Areopagite in the Fourth or Fifth century, in his book The Celestial Hierarchy.In this work, the author drew on passages from the New Testament, specifically Ephesians 6:12 and Colossians 1:16, to construct a schema of three Hierarchies, Spheres or Triads of angels, with each Hierarchy containing three Orders or Choirs. In descending order of power, these were:
First Hierarchy:
Seraphim
Cherubim
Thrones or Ophanim
Second Hierarchy:
Principalities
Virtues
Powers
Third Hierarchy:
Dominions
Archangels
Angels

During the Middle Ages, many other schema were proposed, some drawing on and expanding on Pseudo-Dionysius, others suggesting completely different classifications. Here is another example, which is expanded in the rest of this article:
First Hierarchy:
Seraphim
Cherubim
Archangels
Second Hierarchy:
Dominions
Virtues
Principalities
Rulers
Authorities
Powers
Thrones
Third Hierarchy:
Angel

The angels of the first sphere were thought to serve as heavenly counselors.

Seraphim

The Seraphim (singular "Seraph") are the highest order of angels, serving as the caretakers of God's throne and continuously singing his praises: "Holy, holy, holy is the Lord of hosts. All the earth is filled with His Glory." It is said that they surround the throne of God, singing the music of the spheres and regulating the movement of the heavens as it emanates from God. It is also said that such a bright light emanates from them that nothing, not even other divine beings, can look upon them. There are four of them surrounding God's throne, where they burn eternally from love and zeal for God.

Many Christians believe that the Devil is a fallen angel that was among the Seraphim prior to his fall from Heaven. He was believed to be second in power to God. They believe that the Devil was considered as the angel of light before his sin against God. Unlike other Seraphim who had six wings, Satan was featured to have twelve wings.

The Seraphim are mentioned in Isaiah 6:1–7

The Cherubim (singular "Cherub") are beyond the throne of God; they are the guardians of light and of the stars. It is believed that, although they are removed from man's plane of reality, the divine light that they filter down from Heaven still touches human lives.

They have four wings. They also have four faces, one of man, ox, lion, and eagle. Cherubim are considered the elect beings for the purpose of protection. Cherubim guard Eden and the throne of God.

Their rank among angels is uncertain but they are always categorized in the First Sphere. Some believe them to be an order or class of angels; others hold them to be a class of heavenly beings higher than angels. Cherubim have perfect knowledge of God, surpassed only by the love of the Seraphim.

The Cherubim are mentioned in Genesis 3:24; Ezekiel 10:17–20; and 1 Kings 6:23–28.

The Thrones or Ophanim are angelic beings shaped like shiny orbs of shifting colors. Their duty is to carry the throne of God around in paradise, hence their name.They are the companion angels of the planets.

Also, they have been described as a wheel intersected by another wheel, one to move forward and back, the other to move side to side. These wheels are then dotted with innumerable eyes. This image is found in the book of Ezekiel, where the prophet Ezekiel describes a vision of Heaven. The prophet does not explicitly describe these wheels as angels, but as objects in which the spirits of "living creatures" were contained.

Thrones are fully equal in authority and power in comparison to the Powers. However, Thrones are politically, militarily and economically oriented, and their main concerns revolve around people and world events. Thrones have authority over major divisions of the world.

Angels of the Second Sphere work as heavenly governors.

Dominions

The Dominions, also known as the Hashmallim, hold the task of regulating the duties of lower angels. They receive their orders from the Seraphim, the Cherubim, or God Himself, and are responsible for ensuring that the cosmos remains in order. It is only with extreme rarity that the dominions make themselves physically known to mortals. Instead, they quietly concern themselves with the details of existence.