The word "angel" denotes a function, rather than a nature. Those holy spirits of heaven have always been spirits. They can only be called angels when they deliver some message. Those who deliver messages of lesser importance are called angels; and those who proclaim messages of supreme importance are called archangels. God created angels as angels (Colossians 1:16). They have never been anything else., and people do not become angels when they die.

In many religions, such as Judaism, Christianity and Islam, it is believed that between God and mankind there are intermediary beings, called angels. They are bodiless entities that perform certain tasks for God and are commonly thought of as the messengers of God. Angels are good spirits, unlike their counter-parts, the demons. They are usually portrayed as having a human form, being dressed in long, white clothes, surrounded by a bright light and with long, swanlike wings. There are cases where angels appeared as ordinary men, and were mistaken as such. Muslims say that angels were created from pure bright gems

In the Old Testament angels play a prominent role as the messengers from God. In the Old Testament, the leading demon, Satan, is introduced. But it was not until the New Testament that Satan was portrayed as Lucifer, the first of the fallen angels to rebel against God. In the New Testament, angels are present at all the important events in the life of Jesus. Here, they became more than just messengers; they are portrayed as the agents of God in bringing judgment to the world.

Until the New Testament, there were only two orders of angels; the seraphim and cherubim. The commonly used hierarchy of nine orders is that popularized by the Pseudo-Areopagite of Pseude-Dionysius (early 5th century) in his "De Hierarchia Celesti", which arranges them in three triads: Seraphim, Cherubim, and Thrones (Ophanim) in the first circle; Dominions (Hashmallim), Virtues (Tarshishim), and Powers in the second circle, Principalities, Archangels and Angels in the third circle.

The seven holy angels are Michael, Gabriel, Raphael, Uriel, Chamuel, Jophiel, and Zadkiel. Michael and Gabriel are mentioned in the Bible, Raphael in the Apocrypha and all appear in Enoch. A list of fallen angels is given by Milton in "Paradise Lost"

Angels are mentioned almost 400 times, from beginning to the end of the Bible. When the word "messenger" appears in the Bible, it refers to a human, and the Bible uses the word "Angels" to describe spiritual beings. The Book of Revelations is the only book in which the words "Angels" are used to describe humans.
Rev. 2:1, 8, 12, 18; 3:1, 7, 14

condensed Article courtesy of: eyecandee.com 2003
sources of information:
Encyclopedia Mythica
A Popular Dictionary of Judaism
Brewer's Book of Myth and Legend.
Encyclopedia of Eastern Philosophy and Religion