An angel is a pure spirit created by God. The Old Testament included the belief in angels: the name applied to certain spiritual beings or intelligences of heavenly residence, employed by God as the ministers of His will.
I was surprised to learn that there are apparently nine choirs, or orders, of angels. Seven are mentioned in the New Testament and two in the Old Testament. Here's a briefing on what I learned: (most from "Catholic Online")
Seraphim are the highest order of angels. They are angels who are attendants or guardians before God's throne. They have six wings: 2 cover their face, 2 cover their feet, and 2 are for flying. I can't really even produce the image of a Seraph in my mind. There are also Cherubim, the 2nd highest choir. They are the guardians of God's glory.
Thrones are the Angels of pure Humility, Peace and Submission. They reside in the area of the cosmos where material form begins to take shape. The lower Choir need the Thrones to access God. I am unclear of how we know what we know about Thrones.
Dominion are Angels of Leadership. They regulate the duties of the angels, making known the commands of God. I am unclear of how they access God and how we know anything about this order, either. There are Virtues, known as the Spirits of Motion who control the elements. Apparently this choir controls all nature- the seasons, stars, moon, and even the sun. They are also in charge of miracles and provide courage, valor, and grace.
The most interesting to me were the Powers. Powers are Warrior Angels against evil, defending the cosmos and humans. They are known as potentates. The chief is said to be either Samael or Camael, both angels of darkness. Are they fighters in the sense of Spiritual Warfare?
The Archangels are the most frequently mentioned throughout the Bible. They may be of this or other hierarchies as St. Michael Archangel, who is a princely Seraph. The Archangels have a unique tole as God's messengers to the people at critical times in history and salvation as in the Annunciation and Apocalypse. Of special significance is St. Michael as he has been invoked as patron and protector by the Church from the time of the Apostles.
Okay, the next one confuses me a lot. Principalities are one type of spiritual (metaphysical) being which are now quite hostile to God and to humans. These beings were created through Christ and for Him. Given their hostility to God and humans due to sin, Christ's ultimate rule over them expresses the reign of the Lord over all in the cosmos. This is the Lordship of Christ.
Christ's reign over them is because of his holiness because humans (and Christ was human) are beneath angels (Psalm 8). "God has given him authority over all things (except God, who gave Christ his authority)" is stated in 1 Corinthians 15:27. So Christ has power over the angels. But why would only the Principalities be upset?
Why are they wicked? They are not fallen angels or are they? What is this about? Where do I find more information?
The last group is comprised of the regular angels, the common folk. These angels are closest to the material world and to human beings. They deliver the prayers to God and God's answers and other messages to humans. Angels have the capacity to access any and all other angels at any time.
Now, I am excited to learn that we all have guardian angels. Psalm 91:10-12 states that "no evil will conquer you; no plague will come near your dwelling. For he orders his angels to protect you wherever you go. They will hold you with their hands to keep you from striking your foot on a stone." I have a question about guardian angels. If they are in accord with God, and yet are submissive to us, then do they have free will? Since they would have to choose to go against God? But then, why don't they sin?
There's a lot to take in there. My brain hurts right now. =D But it is definitely interesting! I was thinking about this last night when we were talking about how servers who do incense are called "Thurifer's" and cross bearers are called "crucifer's", so light bearers, or candle bearers, would be called "Lucifer's." So we easily got off topic and started talking about angels. I have been exposed recently to "guardian angels" more and more through my studying, and promote others to become closer to one's own angel. They are always there, to help.