There was once some debate as to how many would fit on a pin head!

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.'" With their whole beings the angels are servants and messengers of God. Because they "always behold the face of my Father who is in heaven" they are the "mighty ones who do his word, hearkening to the voice of his word." As purely spiritual creatures angels have intelligence and will: they are personal and immortal creatures, surpassing in perfection all visible creatures, as the splendor of their glory bears witness. From the Incarnation to the Ascension, the life of the Word incarnate 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. (Cf. Mt 1:20; 2:13, 19; 4:11; 26:53; Mk 1:13; Lk 22:43; 2 Macc 10:29-30; 11:8) Again, it is the angels who "evangelize" by proclaiming the Good News of Christ's Incarnation and Resurrection. (Cf. Lk 2:8-14; Mk 16:5-7) They will be present at Christ's return, which they will announce, to serve at his judgment. (Cf. Acts 1:10-11; Mt 13:41; 24:31; Lk 12:8-9) For more information, see the Catechism of the Catholic Church, sections 328 and following: http://www.usccb.org/catechism/text/pt1sect2chpt1art1p5.htm#328 With love in Christ.

Annie Lennox believed in Angels..."there must be an angel"--well it was a great song :)

In case your wondering what "Spirit" means here's Frank Sheed's explanation from A Map of Life(he was an amazing Catholic Theologian and apologist, has really awesome books). Spirit is being which has the power of knowing and willing. Matter is being which has not these powers. There is a more obvious but less important distinction between them: matter can be perceived by the senses, spirit cannot. Of God's creatures there are some that are pure spirits–angels–with no material part. There are some that are purely material–animals, plants, stones and the rest–with no spiritual part. Between them is man. In him alone spirit and matter are united: by his soul he is a spirit as the angels are: by his body he is part of the material universe.