What are Archangels ?

The ancient Babylonians believed God has attendants who are spiritual beings. Judaism maintained this belief, and thus the Catholic religion teaches that "purely spiritual creatures" called angels exist (Catechism of the Catholic Church 330).

The word angel comes from the Hebrew word malakh, which means "to send". The Bible has many references to different kinds of angels.  Some angers are messengers.  The prefix arch- means "important".  Thus, archangels are entrusted with very important messages from God.  For example, the archangel Gabriel announced to Mary that she was to be the mother of God (Luke 1:26).

Three archangels are mentioned in the Bible.  Biblical writers named them in reference to the messages they delivered: Michael ("who is the God"), Gabriel ("power of God"), and Raphael ("God has healed").  Their feast day is September 29.

Archangels are not human, but they fit the definition of a saint.  Therefore, the Roman Catholic Church added the archangels to the canon (list) of saints, because a saint is anyone who is holy, he or she lives in the presence of God and exists to serve God.

(Text taken from Fr. Don Kirchner, CSsR)