Angel, from the Greek ἄγγελος (angelos, "messenger"), in the Hebrew and Christian Bibles, and the Quran, is a non-human spirit that acts as a messenger of God.
It is believed that there are different kinds of angels arranged in hierarchies, but since the Biblical canon does not offer enough information, these hierarchies are highly speculative.
The most influential Christian classification was the one designed by Pseudo-Dionysius the Areopagite in the 4th or 5th century in his book De Coelesti Hierarchia (The Celestial Hierarchy).
1. Seraphim (singular Seraph) – Isaiah 6:1-7. These angels are closest to God. It is said that such a bright light emanates from them that nothing, not even other angelic beings, can look upon them (the name Seraphim means "the burning ones"). They praise God: "Holy, holy, holy is the Lord of hosts. All the earth is filled with His Glory." There are four of them surrounding God's throne and their primary mission is to protect the throne. They have six wings: two cover their faces, two cover their bodies, and two cover their feet.
2. Cherubim (singular Cherub) – Genesis 3:24 and Ezekiel 10:1-22 - These angels are the second highest in the nine hierarchies of angels. Cherubim guard the way to the tree of life in the Garden of Eden (Genesis 3:24) and the throne of God (Ezekiel 28:14-16). They are fierce angels. When they come to earth, they have great power of apocalypse nature. They have four faces: one of a man, one of an ox, one of a lion, and one of an eagle.
3. Thrones or Elders – Colossians 1:16 – Their name literally means throne or highly ornamented chair. They are living symbols of God's justice and authority, and dispense God’s divine judgment.
4. Dominions – Colossians 1:16 - These are the angels of leadership, regulating the duties of the lower angels. They are also the angels who preside over nations, and give power to heads of government and other authority figures. The Dominions are believed to look like divinely beautiful humans with a pair of feathered wings, much like the common representation of angels.
5. Virtues or "Strongholds" – 1 Peter 3:22 - Their primary duty is to supervise the movements of the heavenly bodies and control the elements to ensure that the cosmos remains in order. They govern all nature and perform miracles.
6. Powers, or "Authorities" – Colossians 1:16 - These angels are the bearers of conscience and the keepers of history. Their duty is to oversee the distribution of power among humankind, hence their name. They also control the border between heaven and earth. The Powers are warrior angels created to be completely loyal to God, who fight against evil. They are the angels of birth and death.
7. Principalities – Colossians 1:16 - These angels appear to collaborate in power and authority with the Powers. Their task is to oversee groups of people. They are in charge of the world’s nations, cities, and towns. Their duties include taking charge of religion, politics, and the duties of the angels on earth below them. They are the educators and guardians of the realm of earth. Like beings related to the world of the germinal ideas, they are said to inspire living things to many things such as art or science.
8. Archangels – 1 Thessalonians 4:16 - These angels are the chief angels, guardians of people and all things physical. They appear only in human form. They have the task of being God’s messenger to people at critical times of need.
9. Angels – Throughout the Bible - These are the lowest order of the angels, and are sent as messengers to mankind. They are the ones most concerned with the affairs of living things, and act as our personal guardian. They deliver prayers to God and God’s answers and messages to us.