Belief in angels is a doctrine of the Catholic Church, attested to by both Sacred Tradition and Scripture. The author of Hebrews asks "Are they not all ministering spirits sent forth to serve, for the sake of those who are to obtain salvation?" We know that angels can be protectors and messengers, but where did they come from?
One misguided notion popular today is that innocent people, particularly infants or young children, somehow become angels when they die. While that can be a comforting thought and may often help with the grieving process, the fact is that human beings do not become angels after death. Angels are a separate entity, do not procreate, and according to the Fourth Lateran Council, were created before the first human beings.
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." We also acknowledge that they are real persons, named by God (no need for us to do it!), but without a physical body. Tradition informs us of nine choirs of angels, grouped in three levels: Seraphim, Cherubim, and Thrones; Dominations, Powers, and Principalities; Virtues, Archangels, and Guardian Angels. As servants and messengers of God they continually behold His face while executing their specific office.
Superior in power and strength to humans, angels possess an angelic soul with intellect and free will. Not having the restriction of a material brain, they acquire knowledge immediately and their decisions are final. Thus when Lucifer along with a third of the created angels chose to reject God and were defeated by Michael (Rev. 12), they incurred eternal damnation with no chance for repentance. Saints Peter and Paul both warn us of the struggle we face with these evil spirits who roam the world while operating within a hierarchy of their own.
Scripture relates numerous instances of angelic encounters, for which angels took the form of humans. But in the first Epistle of John we are instructed "to test the spirits to see if they belong to God", indicating that not all angels or those who claim angelic messages seek our good. For this reason it is important to rely on and adhere to authoritative representatives of the Church to help us discern any perceived encounters with God's purely spiritual creations.