To be added soon: The rationalist view of angels, as held by Maimonides, Gersonides, Samuel Ibn Tibbon, etc.
In the New Testament an angel appears to Mary in the traditional role of messenger to inform her that her child will be the Messiah, and other angels are present to herald his birth.
Angels are frequently depicted as human in appearance, though many theologians have argued that they have no physical existence. (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 6 wings radiating from a center either concealing a body or without a body.
Some Christian traditions hold that there are as many as ten classes of angels; this is particularly clear in the work of Pseudo-Dionysius the Areopagite, an unknown fifth century author whose work The Celestial Hierarchy gives the names that have become part of tradition: angels, archangels, principalities, powers, virtues, dominions, thrones, cherubim, and seraphim.