A cherub (Hebrew כרוב, plural כרובים) is a supernatural angelic being similar to Assyro-Babylonian shedu (human-headed winged bulls). In fact, it seems that the Assyrians may have used the term karabu to describe these beings that seem to have been spirits that served the gods.
The meaning of "cherub" is obscure. It could be related to the word "mount" by an inversion of letters. This would refer to them as being a mount upon which God sits. This sort of imagery is used in the Psalms and Ezekiel - God sitting or riding upon the cherubim.
"and He rode upon a cherub and did fly: and He was seen upon the wings of the wind." (Ps 18:10)The fact that God's presence was "located" above the Ark of the Covenant, over the two cherubim also seems to suggest that the cherubim were "mounts" in every sense of the word.
In Akkadian, the cognate of karabu means "blessed," whereas the cognate in Assyrian means "great, mighty."
Cherubim are first mentioned in the Genesis account when Adam and Eve are exiled from the Garden of Eden. Cherubim were placed at the portal of the Garden with flaming swords. Later the cherubim were depicted in the Temple (which is itself a model of the Garden of Eden). Two statues of cherubim stood at either side of the Ark of the Covenant.
The Akkadian lammasu seem to be cherubim-like creatures. They are lions with human heads and eagle wings (see photo above) and somewhat resembled the Egyptian sphinx.
My own opinion is that cherubim are depicted as a composite of the eagle, human, lion, and ox because these four creatures are at the four corners of the astrological zodiac: Leo (lion), Scorpio (Eagle as the Ophiucus), Aquarius (human), and Taurs (bull). The composite creature symbolizes the cosmos over which God sits enthroned.
So why do we associate fat babies with "cherubs"? This confusion arose during the Renaissance. Technically, little fat babies are not "cherubs", but "putti," an older Italian word meaning "children." So next time you refer to a cut little baby as "cherubic," know that you are actually describing the child as a sword wielding beast with eagle wings and a human head!