In many mythologies and religions, an angel is a good spirit. The word angel comes from the Greek word angelos which means "messenger".  Angels appear frequently in the Old Testament, the New Testament, Qur'an and Aqdas.

Different references to angels throughout the Bible suggest different kinds and ranks of angels, such as seraphs or cherubs. This resulted in medieval theologians outlining a hierarchy of such divine messengers, including not only cherubs or cherubim (the Hebrew plural) and seraphim, but also archangels, powers, principalities, dominions and thrones. 

The study of Angels is called Angelology.

In the Bible 
Angels are powerful, smart spirits that obey God's commands, praise him with singing, and they have a male (masculine)gender, but without any sex. They sometimes appear to humans in a human form. They can deliver messages to people in person or in dreams. Angels that are named in the Bible are Michael (called a "chief prince"), Gabriel (known for telling Mary that she would be the mother of Jesus), and Raphael (in the Apocryphal Book of Tobit).  The Ethiopian Book of Enoch also lists four Archangels which watch over the four quadrants of heaven; Michael, Raphael, Gabriel and Uriel. Lucifer is also known as an angel in the Bible.

Types
Cherubim (plural of cherub) are described as "winged creatures" which have four wings Cherubim guard the Eden with a sword of fire. This suggests that the author of Genesis was aware of different types of angels. Genesis 3:24 is found in the Book of Ezekiel. A Cherub is mentioned in Ezekiel 28:13-14, saying that the angel was in the Garden of God.

Ezekiel 28:13-14
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.

It describes the sound of their wings, "like the roar of rushing waters."

Ezekiel 10:5-7 ; Ezekiel 10:8 reveals that they have hands like a man under their wings .

Ezekiel 1:7 KJV reveals that they look like man but are different because they have "straight feet" and four wings and four faces.

Ezekiel ch 1, and 10 describe the cherubim creatures ascending and descending from the earth with wheels. Ezekiel 1:14-20 ; Ezekiel 10:16

Ezekiel 10:9-13 describes what the wheels appeared to look like, and how they moved around, how they moved or flew through the sky quickly but turned not as they went; and how the inside workings of the wheels appeared to be "a wheel in the midst of a wheel" and that the color of the wheels was the color of "Amber" Stone. There are four separate wheels in both accounts, one for each single cherub which is there.

Seraphim (plural of seraph, which means "burning")they also are depicted having wings, six of them. They are known for singing and praising God. They can shout so loud, they shake the temple.
Archangels like Gabriel (Gospel of Luke 1:19) are the highest type of angel. They are considered saints in the Catholic church. However, in the King James Version of the Bible; they are another type of angel. In the Book of Revelation the Angel Michael casts the 'great dragon' Satan out of heaven and down to earth in a great battle between the good and bad angels, just before the Great Judgement of angels and man. (Revelation 12)
The Leviathan in Book of Job 41:19-21 has flame that goes:  'out of his mouth' like a dragon.  Isaiah 30:6 also talks of a 'fiery flying serpent'. Compare Revelation 20:2: , where an angel: 'laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years'.

In art
They are often shown in art as having wings and a halo. The wings represent their speed, and the halo represents their holiness.

The cherubim in art always appear as baby faced angels with very small, non-useful wings.

The cherubim statue or bronze casting of cherubim in the Temple of Solomon depicted them as two four winged creatures whose wings touched at the peak of the ark that they were making.

The same cherubim creatures were said to be cast in gold on top of the Ark of the Covenant. Casting metal is one of the oldest forms of artwork, and was attempted by Leonardo da Vinci.

In literature 
Angels are generally held to be holy and virtuous, hence the term is used loosely to apply to anyone particularly good or kind, or having a good influence. In his novel Far From the Madding Crowd, Thomas Hardy chooses the name of an angel, Gabriel, for his kind and helpful hero. On the other hand, in his play Measure for Measure, Shakespeare's use of the name Angelo is ironic, since Angelo is a character who likes to see himself as virtuous, but who is concealing evil aspects of his nature. Fallen angels, who are no longer holy or virtuous, are also known as devils.

However, since angels are held to be spirits (that is, non-material beings), medieval theologians were faced with the problem of how humans could see a non-physical creature. Eventually a theory was put forward that angels must make themselves a body out of the nearest thing to the non-physical, i.e. from air. Hence in his famous poem Aire and Angels, the seventeenth century metaphysical poet John Donne uses this idea to write a cynical comment on women, whose love, he says, is like an angel's body of air, while men's love is like the real thing, the angel itself.

Idea of Guardian angel 
From the era of the Romantics onwards, there has developed the widely held belief that everyone has an angel assigned to guard them. This concept is probably based on Jesus' comment in Matthew 18:10 regarding children, though it is not mentioned elsewhere in the Bible.

In superstitions
Seeing repetitive numbers are thought to be associated with numerology, also referred to as angel numbers. It is believed that angels communicate with humans through repetitive appearances of numbers. Humanity has studied and used numbers since the dawn of time, and no matter what the culture is, there are certain numbers that hold specific value or meaning over other numbers.

References

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