The word "angel" comes from the Greek word angelos, which means "messenger". They are engaged in God's service to perform specific divine tasks. While they do not have material bodies, they often appear in human form as men or as children but never as women.

In chiristian tradition all angels are not known by name only by the function assigned them both in the Bible and by St. Paul, but the arrangement in hierarchies was given by Dionysius the Pseudo-Areopagite around 500 AC. In his " Celestial Hierarchy" he devided the celestial court into three groups of three, means nine choirs of angels devided into three ranks:

1. Councillors (Seraphim, Cherubim and Thrones)
Their function is to prise and worship God and they constitute the highest order in the hierarchy where:
The Seraphim represent Devine Love, closely located around the throne of God, usually artists following description in Book of Isaiah in the Old Testament portrayed them as a six winged angel (one pair for flying, another covering the face and the third pair covering the feet) in red (symbolizing fire), sometimes holding candles in their hands.
The Cherubim represent Divine Wisdom. Usually portreyed as four winged angel in golden yellow or blue (symbolizing sky) sometimes holding books in their hands. They know God, worship Him and sing His glory in choirs consists of four, six, or seven angels. A four angels choir symbolically implies perfection and is related to the four cardinal points. In Rococo a Cherubim choir is usually depicted as four singing putti.
The Thrones represent Devine Justice, closely standing in the glory of God , usually portrayed in robes of judges carrying the staff of authority in their hands.

2. Governors (Dominations, Virtues, Powers)
The second group in the hierarchy is made up of the regents and governors of the stars and the elements where:
The Dominations represents the Power of God and usually portrayed crowned, carrying scepters, sometimes orbs as emblems of their authority.
The Virtues represents Christ's Passion, usually carrying white lily as a symbol of purity and sometimes red rose as a symbol of Passion.
The Powers are warriors against the hordes of evil devils, usually portrayed in full armor.

3. Ministers (Princedoms, Archangels and Angels)
The third group in hierarchy is made up of the executors of the Will of God. In relation to men the Princedoms are the dispensers of the fate of nations, the Archangels are warriors of Heaven and the Angels are responsibile for protecting and guiding men.
God "has charged his angels to guard you wherever you go, to lift you on their hands for fear you should strike your foot against a stone" (Psalm 91:11)

In the last group 4 Angels, special tasks ministers of God to man, are known by name Michael, Gabriel, Raphael and Uriel. Michael and Gabriel are mention by name in the books of the Bible, Raphael and Uriel are known from apocrypha "Book of Henoch" where their functions and responsibilities are revealed wider and more detailed.

Michael whose name means "Who is like God?" is invested with authority from God Capitan-General of Heaven and a Protector of the Church.
Christian tradition portrayed him as a young and hansom warrior with large white or black wings rising from his shoulders in dazzling armor, holding a sword, spear and shield in his hand fighting with Satan who is represented as dragon, serpent or demon. This refers to dramatic description in Book of Revelation: And there was war in Heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels, and prevailed not, neither was their place found any more in heaven. And the great dragon was cast out that old serpent called the Devil, and Satan, which deceived the whole world: he was cast out into the earth and his angels were cast out with him. " This explains also genesis of the Satan and why he is sometimes called "fallen Angel".

Gabriel whose name means "Mighty one of God" or "God is my strength" appears as God's special messenger of His kingdom program. He is a guardian of the celestial treasury, the Angel of Redemption and he is the Angel of the Annunciation usually portrayed as a young, beautiful, majestic figure with large white wings dressed in richly robe embroidered in white lilies - symbol of the purity of the Virgin, sometimes wears a crown and bears a scepter to indicate sovereignty.

Raphael whose name means "the Medicine of God" is chief of the guardian angels and the guardian angel of all humanity, protector of the young and innocent, especially pilgrims and wayfarer. He is usually shown as a kind, mild and loving person dressed in the richly traveler robe, wears sandals but burley touching the his path, almost dancing in the air, his curly hair is bound with a diadem. Usually he carries a medicine, a cure for illness, scroll or fish in his hand , and there is sometimes a gourd of water or a wallet slung to his golden belt. According to the Bible he was the protector of Tobias on his journey to collect money of his blind father. According to the Gospel he was the Angel who appeared to the shepherds on Christmas night.

Uriel, whose name means "the Light of God" is portrayed much less frequently than are Gabriel, Michael or Raphael. Early legend states that it was Uriel who, as ambassador of Christ, appears to the disciples at Emmaus. He is usually shown as a massager carrying a scroll and a book indicating his role as interpreter of judgments and prophecies. The open book represents truth or revelation of the New Testament. Old Testament writings are usually represented by a scroll.

From beginning of the nineth century Dionysius's hierarchical arrangement of angels become the standard text till 13th century when some minor modification were done by Dante. In his "Paradiso" ( 28: 97-124) Dante also added names of the angels known from Apocrypha and from various Jewish writings - Uriel, Raguel, Ramuel, Sariel, Jeramial, Chamuel, Jophiel, and Zadkiel to the angels known by name from the Bible, Gabriel, Michael, and Raphael.

However, angels primary function is to praise and serve God to do His will, reveal divine truth, assist man in achieving a proper rapport with God, singing they also are believed to be efficacious in helping man to attain salvation, or special graces, or favors. God has charged his angels to guard you wherever you go, to lift you on their hands for frear you should strike your foot against a stone" (Psalm 99:11) . Although he concept of guardian angel was known from Psalms, it become popular not earlier than at the end of XVIth century and become universal text by pope Clement X in 1670 AC. Thus the guardian angels are believed to be guardians over individuals and nations. They help man in everyday life, watch him and prevent him to offend against God.