Catechism of the Catholic Church
E. Para. 350: Angels are spiritual creatures who glorify God without ceasing and who serve his saving plans for other creatures: “The angels work together for the benefit of us all” (St. Thomas Aquinas, STh I, 114, 3, ad 3).
Para. 329: St. 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’ (St. Augustine, En. in Ps.103, 1, 15: PL 37, 1348). With their whole beings the angels are servants and messengers of God.