A cherub or cherubim plural, is a Biblical 4-faced heavenly creature that sits at the throne of God. Four parts include: a lion, an ox, an eagle, and a man. There are slight variations to the purpose of cherubim based upon faith.

Jewish
In the Jewish angelic hierarchy a cherub is the 9th highest angel. (Catholic theory puts cherubim as the 2nd highest) The destroyed Jerusalem Temple had a cherub statute in it according to Yoma 54b. 2 Chronicles confirms this, see below. Yoma 54a says the Herod Temple had drawings of cherubim on the walls.

King James Bible References
Cherubim are found in the Holy of Holies, on the Mercy Seat of the Ark (Ex. 25: 18, 22; 1 Kgs. 6: 23-28; Heb. 9: 5), and in the visions of Ezekiel. In Revelations cherubim declare each of the 7 Seals are broken. In Genesis, two cherubim protect the Tree of Life in the Garden of Eden. Many scriptures indicate God put the cherubim there before Adam was on earth, thus suggesting that Adam would have had to pass by, and possibly see, the cherubim to partake of the fruit. Most Biblical references reveal that cherubim come in pairs. Another example is 2 of them at the mercy seat.

There is no direct explanation in the Scriptures about the meaning of the four faces. Here's a theory from the Catholic Encyclopedia. "The mystical meaning of each detail of this vision (referring to Ezekiel's vision) will probably remain a matter of speculation, but the meaning of the four faces seems not difficult to grasp: man is the king of creation, the lion the king of beasts of the forest, the ox the king of the kine in the field, the eagle the king of the birds of the air. On this account the cherubim have of recent years been explained as mere symbols of the fulness of earthly life, which, like the earth itself, is the footstool of God. But these faces are more naturally understood to signify that these angelic beings possessed the intelligent wisdom of man, the lithe strength of the lion, the ponderous weight of the ox, the soaring sublimity of the eagle." (New Advent)

Description

I find this to be accurate based upon my readings, "They are usually clothed in blue, while the seraphim are clothed in red. They originated from the winged and human-headed bulls of Babylon (also named cherubim), a lesser order of deities, which guarded the gates of the royal palace." (Pantehon.org)

Ezekiel describes a cherub as a "tetrad of living creatures, each having four faces: of a lion, an ox, an eagle, and a man." For a picture of the faces click here. In Ezekiel chapater 1, he describes a cherub to have four sets of wings and hands of a man.

Ezekiel

    Ezekiel 1: 5-14
    5 Also out of the midst thereof came the likeness of four living creatures. And this was their appearance; they had the likeness of a man.
    6 And every one had four faces, and every one had four wings.
    7 And their feet were straight feet; and the sole of their feet was like the sole of a calf’s foot: and they sparkled like the colour of burnished brass.
    8 And they had the hands of a man under their wings on their four sides; and they four had their faces and their wings.
    9 Their wings were joined one to another; they turned not when they went; they went every one straight forward.
    10 As for the likeness of their faces, they four had the face of a man, and the face of a lion, on the right side: and they four had the face of an ox on the left side; they four also had the face of an eagle.
    11 Thus were their faces: and their wings were stretched upward; two wings of every one were joined one to another, and two covered their bodies.
    12 And they went every one straight forward: whither the spirit was to go, they went; and they turned not when they went.
    13 As for the likeness of the living creatures, their appearance was like burning coals of fire, and like the appearance of lamps: it went up and down among the living creatures; and the fire was bright, and out of the fire went forth lightning.
    14 And the living creatures ran and returned as the appearance of a flash of lightning.

Ezekiel chapter 10 is repetitive in nature to chapter 1, with the exception of the wheel explanation, which I find to be representative/symbolic to the throne of God. 10:9 And when I looked, behold the four wheels by the cherubims, one wheel by one cherub, and another wheel by another cherub: and the appearance of the wheels was as the colour of a beryl stone.

Besides Ezekiel, reference to Cherubim are widspread throughout the Old and New Testament. Below are a selection of some of those references.

Genesis

In Genesis, God places two cherubim to protect the Garden of Eden.

    Genesis 3:24
    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.

Numbers

Numbers 7:89 is a conversation between God and Moses atop the Ark of the Covenant. Notice the location reference of the mercy seat or seat of grace (Exodus 37 also mentions the mercy seat).

    Numbers 7:89
    89 And when Moses was gone into the tabernacle of the congregation to speak with him, then he heard the voice of one speaking unto him from off the mercy seat that was upon the ark of testimony, from between the two cherubims: and he spake unto him.

2 Samuel

Many passages suggest God sits upon a throne of cherubims. This particular verse indicates God rides them like a mount.

    2 Samuel 22:11
    11 And he rode upon a cherub, and did fly: and he was seen upon the wings of the wind.

Psalms

God sits on a throne between cherubims. See also Psalms 80:1 and Isaiah 37:16.

    Psalms 99: 1
    1 The Lord reigneth; let the people tremble: he sitteth between the cherubims; let the earth be moved.

1 Kings

This passage talks about the size of a cherub. A cubit is 18 inches, so 10 cubits would be 15 feet tall, the height.

    1 Kings 6: 24-27
    24 And five cubits was the one wing of the cherub, and five cubits the other wing of the cherub: from the uttermost part of the one wing unto the uttermost part of the other were ten cubits.
    25 And the other cherub was ten cubits: both the cherubims were of one measure and one size.
    26 The height of the one cherub was ten cubits, and so was it of the other cherub.
    27 And he set the cherubims within the inner house: and they stretched forth the wings of the cherubims, so that the wing of the one touched the one wall, and the wing of the other cherub touched the other wall; and their wings touched one another in the midst of the house.

2 Chronicles

Cherub statutes were erected in the Temple of Solomon. Specifically inside the inner temple and the holy of holies. One important thing to notice in this passage as well as Exodus 37 is how the faces are turned inward.

    2 Chronicles 3:10-14
    10 And in the most holy house he made two cherubims of image work, and overlaid them with gold.
    11 ¶ And the wings of the cherubims were twenty cubits long: one wing of the one cherub was five cubits, reaching to the wall of the house: and the other wing was likewise five cubits, reaching to the wing of the other cherub.
    12 And one wing of the other cherub was five cubits, reaching to the wall of the house: and the other wing was five cubits also, joining to the wing of the other cherub.
    13 The wings of these cherubims spread themselves forth twenty cubits: and they stood on their feet, and their faces were inward.
    14 ¶ And he made the vail of blue, and purple, and crimson, and fine linen, and wrought cherubims thereon.

Exodus

Notice the inward faces here as well as the location reference of the mercy seat.

    Exodus 37:7-9
    7 And he made two cherubims of gold, beaten out of one piece made he them, on the two ends of the mercy seat;
    8 One cherub on the end on this side, and another cherub on the other end on that side: out of the mercy seat made he the cherubims on the two ends thereof.
    9 And the cherubims spread out their wings on high, and covered with their wings over the mercy seat, with their faces one to another; even to the mercy seatward were the faces of the cherubims.

Hebrew

Most of this chapter explains how Mosaic ordinances prefigured Christ's ministry. Verse 5 is yet another reference to cherubim next to the mercy seat.

    Heberew 9:3-5
    3 And after the second veil, the tabernacle which is called the Holiest of all;
    4 Which had the golden censer, and the ark of the covenant overlaid round about with gold, wherein was the golden pot that had manna, and Aaron’s rod that budded, and the tables of the covenant;
    5 And over it the cherubims of glory shadowing the mercyseat; of which we cannot now speak particularly.

More Ezekiel

Another Garden of Eden reference, but more importantly notice the possibility of anointing a cherub. This suggests that there is an angelic hierarchy. This passage is Ezechiel's Prophecy against the Prince of Tyre and contains a description of the almost more than earthly glory of that ancient city. Tyre is spoken of as an angel fallen from glory. "Indirectly we can gather from this passage that Cherubim were conceived to be in a state of perfection, wisdom, sinlessness, nearness to God on His Holy Mountain and of preternatural glory and happiness. Unfortunately, the words paraphrased as 'with wings outstretched in protection' are difficult to translate: the Hebrew term may mean 'cherub of anointing, who covers', therefore a royal, anointed being, overshadowing others with its wings to shelter them. If this be so, we must add royalty and beneficence to the characteristics of cherubim." (New Advent)

Also note that Mountain of God is often symbolic to mean Temple.

    Ezekiel 28:13-16
    13 Thou hast been in Eden the garden of God; every precious stone was thy covering, the sardius, topaz, and the diamond, the beryl, the onyx, and the jasper, the sapphire, the emerald, and the carbuncle, and gold: the workmanship of thy tabrets and of thy pipes was prepared in thee in the day that thou wast created.
    14 Thou art the anointed cherub that covereth; and I have set thee so: thou wast upon the holy mountain of God; thou hast walked up and down in the midst of the stones of fire.
    15 Thou wast perfect in thy ways from the day that thou wast created, till iniquity was found in thee.
    16 By the multitude of thy merchandise they have filled the midst of thee with violence, and thou hast sinned: therefore I will cast thee as profane out of the mountain of God: and I will destroy thee, O covering cherub, from the midst of the stones of fire.

In chapter 9 Ezekiel sees the identifying of the righteous and the slaughter of all others, beginning at the Lord’s sanctuary.

    Ezekiel 9:3 3 And the glory of the God of Israel was gone up from the cherub, whereupon he was, to the threshold of the house. And he called to the man clothed with linen, which had the writer’s inkhorn by his side;

Revelations

Note the reference to 6 wings, instead of 4 in this passage.

    Revelations 4:6-9.
    6 And before the throne there was a sea of glass like unto crystal: and in the midst of the throne, and round about the throne, were four beasts full of eyes before and behind.
    7 And the first beast was like a lion, and the second beast like a calf, and the third beast had a face as a man, and the fourth beast was like a flying eagle.
    8 And the four beasts had each of them six wings about him; and they were full of eyes within: and they rest not day and night, saying, Holy, holy, holy, Lord God Almighty, which was, and is, and is to come.
    9 And when those beasts give glory and honour and thanks to him that sat on the throne, who liveth for ever and ever,

Seraphim
While a cherub has 4 wings, a seraphim has 6. In Catholic Theory the seraphim is the only angel higher in the angel hierarchy.

Misinformation
There is a lot of misinformation about cherubim. For starters, baby angels with wings ARE NOT cherubim. Instead those are Puttos or Putti. This mixup has a lot do with the Renaissance and the evolving art depictions. A putto is an innocent soul. Also note the belief in puttos is not nearly as widespread as cherubim, Catholicism being the main believers.

There is a four beast reference in Daniel chapter 7. However, these beasts are Daniel 7:3 "diverse one from another" and are not a cherub. Instead they are a symbolic reference to the 4 kingdoms of men.

    Daniel 7:4-7
    3 And four great beasts came up from the sea, diverse one from another.
    4 The first was like a lion, and had eagle’s wings: I beheld till the wings thereof were plucked, and it was lifted up from the earth, and made stand upon the feet as a man, and a man’s heart was given to it.
    5 And behold another beast, a second, like to a bear, and it raised up itself on one side, and it had three ribs in the mouth of it between the teeth of it: and they said thus unto it, Arise, devour much flesh.
    6 After this I beheld, and lo another, like a leopard, which had upon the back of it four wings of a fowl; the beast had also four heads; and dominion was given to it.
    7 After this I saw in the night visions, and behold a fourth beast, dreadful and terrible, and strong exceedingly; and it had great iron teeth: it devoured and brake in pieces, and stamped the residue with the feet of it: and it was diverse from all the beasts that were before it; and it had ten horns.

Book of Mormon

To my Mormon readers, you will also enjoy reading cherubim references in: Alma 12: 21-29; Alma 42: 2-3; and Moses 4: 31. As well as seraphs/seraphim references in 2 Ne. 16: 2; D&C 38: 1; and D&C 109: 79. The Book of Mormon backs up the Bible's 4 winged cherub and 6 winged seraph differences. While the Mormon faith believes in an hierarchy of angels, it does not list the order or place the importance of the cherub beyond its specified duties as protector of the tree of life, being found in the Holy of Holies, and on the Mercy Seat of the Ark.