Angels in the Bible
In the Christian religion, angels are spiritual beings who serve God. The Bible teaches that angels were created by God, but it doesn't say when they were created. They are depicted as praising God at the time of creation, which suggests that they existed prior to that moment (cf. Job 38:4-7).
The Greek word translated into English as "angel" simply means messenger, which describes the role they have as God's servants (cf. Heb. 1:14). More specifically, the Bible depicts angels bringing messages to God's people (e.g. Luke 1:26-27), protecting God's people (e.g. Psalm 91:11), and bringing judgment upon God's enemies (e.g. Matt. 13:49-50).
Many people today are interested in guardian angels and whether or not the Bible teaches that God assigns every person, or every believer, a guardian angel. While some Bible verses seem to suggest special angelic assignments to individuals (e.g. Matt. 18:10), and churches (e.g. Rev. 2-3) and nations (e.g. Dan. 10:21), most scholars reject the idea that there is an angel assigned to every person.
Beliefs in Angels
According to a Gallup poll, belief in angels and demons is on the rise in the United States. In 1994, 72% of Americans said they believed in angels; in 2004, 78% indicated belief in angels. Belief in the devil has risen even more dramatically, increasing from 55% in 1990 to 70% in 2004.  Another Gallup poll showed that 56% of Canadians and 36% of Britons believe in angels, while only 37% of Canadians and 29% of Britons believe in the devil. 
Angels and Guardian Angels in the New Testament
In the New Testament, angels appear as the ministers of God and the agents of revelation (e.g. in Matt. 1:20 to Joseph, in Matt. 4:11 to Jesus, in Luke 1:26 to Mary, in Acts 12:7 to Peter); and Jesus speaks of angels as fulfilling such functions (Mark 8:38, 13:27), implying in one saying that they neither marry nor are given in marriage (Mark 12:25). Angels are most prominent in the book of Revelation.
The New Testament only hints at the idea of the angelic hierarchy (e.g. Michael is the archangel), although more elaborate forms of their organization appeared later in Christian history. The distinction of good and bad angels is recognized in Scripture; for example, a good angel is Gabriel (cf. Luke 1:19).
Examples of evil angel include Abaddon or Apollyon (Rev. 9:11), Beelzebub (Mark 3:22) and Satan (Mark 1:13); ranks are implied: archangels (Michael, Jude 9), principalities and powers (Rom. 8:38; Col. 2:10), thrones and dominions (Col 1:16). Angels also appear together, such as in groups of four or seven (Rev 7:1). (Learn more about demons here.)
In Revelation 1-3 we meet with the "Angels" of the Seven Churches of Asia Minor. These may be guardian angels, standing to the churches in the same relation that the "princes" in Daniel stand to the nations; practically the "angels" are personifications of the churches. A minority view is that the "angels" are the human representatives of the churches, the bishops or chief presbyters.
Other angelic appearances in Scripture: The angel Gabriel appeared to Mary in the traditional role of messenger to inform her that her child would be the Messiah, and other angels were present to herald his birth. An angel appeared at Jesus' tomb, frightened the Roman guards, rolled away the stone from the tomb, and later told the myrrh-bearing women of Jesus' resurrection. Two angels witnessed Jesus' ascent into Heaven and prophesied his return. When Peter was imprisoned, an angel put his guards to sleep, released him from his chains, and led him out of the prison. Angels fill a number of different roles in the book of Revelation. Among other things, they are seen gathered around the Throne of God singing the "Holy, holy, holy" hymn (Rev. 4-5).
Artistic and Literary Depictions of Angels in Christianity
In art angels are frequently depicted as human in appearance, though many theologians have argued that they have no physical substance. (Hence the frequently recounted tale of Scholastics arguing about how many angels could fit on a pinhead; if angels possess physical bodies, the answer is "a finite number", if they do not, the answer is "an infinite number".) Seraphim are often depicted as six wings radiating from a center — either concealing a body, or without a body.
Beginning in the end of the 4th century, angels were depicted with wings, presumably to convey the idea of swift movement and travelling to and from heaven, or to depict them as spirits. Scholastic theologians teach that angels are able to reason instantly, and to move instantly. They also teach that angels are intermediaries to some forces that would otherwise be natural forces of the universe, such as the rotation of planets and the motion of stars. Angels possess the beatific vision, or the unencumbered understanding of God (the essence of the pleasure of heaven). Furthermore, there are more angels then there are anything else in the universe (although when first written this would have probably not included atoms since atomic structure was not known).
Religious thought about the angels during the middle ages was much influenced by the theory of the angelic hierarchy set forth in the The Celestial Hierarchy, written in the 5th century in the name of Dionysius the Areopagite. The creeds and confessions do not formulate any authoritative doctrine of angels; and modern rationalism has tended to deny the existence of such beings, or to regard the subject as one on which we can have no certain knowledge. The principle of continuity, however, seems to require the existence of beings intermediate between man and God.
Some Christian traditions hold that angels are organized into three major Hierarchies which are subdivided into orders or Choirs, and list as many as ten orders of angels. This is particularly clear in the above-mentioned The Celestial Hierarchy, which gives the names that have become part of tradition: Angels, Archangels, Principalities, Powers, Virtues, Dominions, Thrones, Cherubim, and Seraphim. In this hierarchy, the Cherubim and Seraphim are typically closest to God, while the Angels and Archangels are most active in human affairs. Many of these names come from verses in the Bible which would appear at first to be referencing a literal thing, although retroactively suggesting that they really mention angels can also make sense in the context. For example the verse in Paul "our struggle is not with earthly things but with principalities and powers" (meaning, according to most theologians, the fallen angels of those choirs, used as an example of all the fallen angels).
Some Christian traditions also hold that angels play a variety of specific roles in the lives of believers. For instance, each Christian may be assigned a guardian angel at their baptism (although never defined by the Catholic or Orthodox churches, this is personally held by many church members and theologians). Each consecrated altar has at least one angel always present offering up prayers, and a number of angels join the congregation when they meet to pray. In the story of the 40 martyrs of Sebaste, in which 40 Christian Roman soldiers were made to stand naked on a frozen lake in the snow until they renounced their faith, angels were seen descending from Heaven placing the crowns of martyrs on their heads.
In many informal folk beliefs among Christians concerning the afterlife, the souls of the virtuous dead ascend into Heaven to be converted into angels. However, this belief is not supported by the Bible and theologians are quick to discount it. 3Sources:
- "Eternal Destinations: Americans Believe in Heaven, Hell." Gallup Poll News Service, May 25, 2004.
- "Divine Subjects: Canadians Believe, Britons Skeptical." Gallup Poll News Service, November 16, 2004.
- Angels - Catholic Encyclopedia (1907)
- Early Christian Representations of Angels - Catholic Encyclopedia (1907)
Protestant Sources on Angels
- Christian Answers: What does the Bible teach about angels? - A thorough FAQ
- Angels, Demons and Satan - by Dan Corner of Evangelical Outreach
- Martin Luther's Table Talk on Angels - CCEL
Orthodox Sources on Angels
- The World of the Angels - reprinted from "Orthodox Russia" (1999)
- Angels and Evil Spirits - Orthodox Church in America