Yesterday was the feast day of Sts Raphael, Michael, and Gabriel. Pope St Gregory the Great explains:
You should be aware that the word “angel” denotes a function rather than a nature. Those holy spirits of heaven have indeed always been spirits. They can only be called angels when they deliver some message. Moreover, those who deliver messages of lesser importance are called angels; and those who proclaim messages of supreme importance are called archangels. And so it was that not merely an angel but the archangel Gabriel was sent to the Virgin Mary. It was only fitting that the highest angel should come to announce the greatest of all messages.
Some angels are given proper names to denote the service they are empowered to perform. In that holy city, where perfect knowledge flows from the vision of almighty God, those who have no names may easily be known. But personal names are assigned to some, not because they could not be known without them, but rather to denote their ministry when they came among us. Thus, Michael means “Who is like God”; Gabriel is “The Strength of God”; and Raphael is “God’s Remedy.”