Who they are
The Catechism of the Catholic Church says that the existence of the angels is a truth of faith and the sacred writings testimony their live, as many events about them are described in the Sacred Text.
Saint Augustine says
Although we do not see the presence of angels, as it is something that escapes our eyes and exists in the great kingdom of God Emperor, we know by faith that the angels exist, we can read that they appeared to many people and we believe it beyond any legitimate doubt. Now the angels are spirits, but as spirits they are not angels: they become angels when they are sent. The word angel in fact refers to the office, not to the nature. If you ask for the name of the nature, the answer will be spirit; if you ask for the office, the answer will be angel.
The angelic world was created before the earthly world, but in relationship with it; the angels are spiritual beings without a body and their minds focus the irradiation of the principle creator, they are forces, which are acting both from the outside and inside the world even on the being itself.
Even if the man is at a lower step in the cosmic hierarchy at the centre of a declined world, he can access the highest knowledge of God, even beyond the angelic knowledge, but the angels are accessible to the man, as they are creatures, so God invests them of his wisdom and his glory, they look at humanity with love, infinite tenderness, innocence of a child.
Many people say that you do not need any mediation to come to God; however an enlightened way is easier than a dark one. Many people cannot enjoy this heavenly food, so they lost hope and give up on the way.
Saint Thomas - from "the Somma Teologica".
The angel may be in a place, but it is not limited like the bodies; it is there because it is acting there: so, it is not contained by the place, but it contains the place.
The angel does not know everything by its nature, that is, by himself, because it should have the entire knowledge in itself, and this is up to God: the angel knows as God sends it images, it reflects the knowledge of God.
The higher angels as much perfect and like God are; so much they know more universal images.