A cherub (Hebrew כרוב; plural cherubim, כרובים) is an angelic creature mentioned several times in the Tanakh, or Old Testament, and in the Book of Revelation.
In medieval Catholic theology the Cherubim are one of the highest ranks in the hierarchy of angels, along with Seraphim. In popular Christian tradition, "cherub" and "cherubim" have become synonyms of "angel(s)" and especially with "baby angel(s)". Because some English speakers are unfamiliar with Hebrew plural formation, the word cherubims is sometimes incorrectly used as a plural.
Cherubim in the Bible
Descriptions in the Bible vary, but in general all describe cherubim as winged creatures combining human and animal features. In the book of Genesis cherubim are described as guarding the way to the Tree of Life, east of the Garden of Eden armed with flaming swords (Genesis 3:24): "So he drove out the man; and he placed at the east of the garden of Eden Cherubims, and a flaming sword which turned every way, to keep the way of the tree of life."
Exodus 26:1 attests that cherubim were embroidered on the curtains of the tabernacle. Two olivewood sculptures of cherubim plated with gold, ten cubits high, stood wingtip-to-wingtip guarding the Ark of the Covenant and two further sculpted cherubim are described as standing on the cover of the Ark facing each other (Ex 25:18). The Ark of the Covenant stood in the Holy of Holies, where the glory of God was said to reside; for this reason God is referred to in the Tanach as "God who dwells between the cherubim".
At an earlier period, when Jehovah was still conceived as making physical appearances, the cherubim formed his living chariot, possibly identical with the storm_winds (Psalms xviii. 11; 2 Samuel xxii. 11): "And he rode upon a cherub and did fly: and he was seen upon the wings of the wind ".
Ezekiel documents a different version of cherubim, probably of popular origin (according to the compilers of the Jewish Encyclopedia). The cherubim in this tradition had each four faces— that of a lion, an ox, an eagle, and a man— and combined features of these four creatures, the stature and hands of a man, the hooved feet of a calf (compare the image of Satan), and the two pairs of wings that identified deities, e.g. in contemporary Assyria. Christians will recognize these as the symbols of the four Evangelists. Two of the wings extended upward, meeting above and sustaining the throne of God; while the other two stretched downward and covered the creatures themselves. They never turned, but went "straight forward" as the wheels of the cherubic chariot, and they were full of eyes "like burning coals of fire" (Ezekiel i:5 - 28; ix:3, x; xi:22).
Cherubim in Christian imagery
The conception of angels derived from Biblical descriptions is difficult to present as a visual image, and furthermore composite beings are largely alien to the central Greco-Roman tradition. (Contrast archaic and exotic beings like Harpy, Typhon, Centaur Gryphon etc.) Instead, Christians adopted the image of the lovely winged dawn goddess Aurora or Eos to represent angels.
Cherubim, in particular, are frequently represented as infants (Italian putti) in Christian iconography and Christian_inspired art, as can be seen in innumerable church frescoes and in the work Renaissance painters such as Raphael. The image was often reduced to the head and wings  (http://www.misericordiamss.it/chiesa/dipinti.htm)  (http://www.insecula.com/oeuvre/photo_ME0000033435.html)  (http://expositions.bnf.fr/renais/grand/006.htm).
Christian novelist Madeline L'Engle depicted a cherubim (who referred to itself as such, in the singular) as one of the principal characters in her children's fantasy novel A Wind in the Door. Her verbal description of the creature corresponds, not very precisely, to that given by Ezekiel.
Cherubim in Islam
Muslim traditions narrated in the Hadith literature describe how Muhammad ascended to heaven on the back of Buraq, a human-headed winged horse. This sounds very like a cherub; however, the Qur'an neither mentions any such beings, nor describes angels in this way.
Origin of the word
The etymology of the word cherub is very doubtful. Some scholars connect it with the Babylonian word karabu, meaning to be propitious or blessed. Others connect it with kirabu, the name of the Assyrian winged-bull god. Some scholars have even suggested tentatively that the Greek word gryphon might be derived from cherub.