We know about angels from Scripture
Angels are mentioned very often in the Old and New Testament, and some of the angels are mentioned by name. The nine "choirs" of angels are Seraphim, Cherubim, Thrones, Dominations, Principalities, Powers, Virtues, Archangels, and Angels. The exact number of angels is unknown, but Scripture indicates that their number is great.
St. Augustine
distinguishes between their office and nature
Saint Augustine says: "Angel is the name of their office, not of their nature. If you seek the name of their nature it is 'spirit'; if you seek the name of their office it is 'angel': from what they are, 'spirit,' from what they do, 'angel.'"
God created the angels as pure spirits
When God created the angels he bestowed on them supernatural grace by which they could gain eternal happiness. He gave them great wisdom, power and holiness. They were given the opportunity to merit the reward of heaven by remaining faithful to God. Some of the angels sinned. We do not know the exact nature of the test to which God put the angels.
Christ is the center of the angelic world.
They are His angels: "When the Son of man comes in his glory, and all the angels with him...." (Mt 25:31) They belong to him because they were created through and for him (Col.1:16).
Angels were created first as our world was created
Throughout the history of salvation they announce God's plan for salvation. The Old Testament tells us they closed the earthly paradise, protected Lot, saved Hagar and her child, stayed Abraham's hand, communicated the law by their ministry, led the People of God, announced births and callings, and assisted the prophets, to mention a few examples. The angel Gabriel announced the birth of John the Baptist and that of Jesus himself.
Throughout the lifetime of Jesus they ministered to Him.
From the Incarnation to the Ascension, the life of the Word incarnate (Jesus) is surrounded by the adoration and service of angels. When God "brings the firstborn into the world, he says: 'Let all God's angels worship him.'" (Heb.1:6) Their song of praise at the birth of Christ has not ceased resounding in the Church's praise: Glory to God in the highest!" (Lk 2:14) They protect Jesus in his infancy, serve him in the desert, strengthen him in his agony in the garden, when he could have been saved by them from the hands of his enemies as Israel had been. Again, it is the angels who "evangelize by proclaiming the Good News of Christs Incarnation and Resurrection. They will be present at Christ return, which they will announce, to serve at his judgement.
The Angels and the Church
The whole life of the Church benefits from the mysterious and powerful help of angels. In the Mass, the Church joins with the angels, at the "Holy,Holy, Holy," to adore God. The Church invokes their assistance in the funeral liturgy. "May the angels lead you into paradise...." From its beginning until death, human life is surrounded by the care and intercession of the angels. Each person has a personal protector or guardian angel.
There was a War in Heaven
The Book of Revelation tells us that "war broke out in heaven; Michael and his angels battled against the dragon. The dragon and his angels fought back, but they did not prevail, and there was no longer any place for them in heaven. The huge dragon, the ancient serpent, who is called the Devil or Satan, who deceived the world, was thrown to the earth, and its angels were thrown with it...." (Rev. 12:7)
