The Qur'an was revealed to the Prophet Muhammad over a number of years in the 7th century AD, purportedly through the angel Gabriel. The scripture is organized (roughly) according to the length of the chapters. There are 114 suras (or chapters), with many of the later and more warlike ones appearing near the beginning. The Prophet's first revelations were received in Mecca. He was exiled from Mecca after commanding the people to repent of polytheism and join the One True God, (in Arabic, his name is Allah). The Prophet Muhammad then travelled to Medina and converted the entire city. He led them to a victory over the polytheists at Mecca.
The verses are of two kinds: Meccan and Medinan. Meccan verses are characterized by praise of a monotheistic God and condemnation of polytheism. Medinan verses reflect divine laws and commandments that would help Muhammad in his new role as a leader of men; that is to say, the verses are concerned more with the social laws and obligations of Muslims.
The Qur'an has influenced a great number of people over the centuries, especially in the Middle East. (A link to the text of the Qur'an and translations can be found below.)
Several commentaries on the Qur'an exist. These are labelled as "tafsir" (the word for "commentary.") Allah himself is regarded as the greatest Tafsir. Muhammad is the second-best (his tafsir is called "hadith," the Words of the Prophet). The Companions of the Prophet also produced their own tafsirs. It is important to note that "tafsir" is not scripture; it is a companion to understanding scripture.
It is also important to note that Muhammad is not worshiped as a God. The Muslims are monotheistic. The Qur'an states that God has no equals, nor was he begotten by a greater being, nor has he begotten any other being. Muslims may occasionally view Jesus and the Holy Spirit as infringing on God's divinity. The Christian doctrine of the Holy Trinity is regarded as tantamount to polytheism by Muslims.
The Qur'an was revealed to the Prophet Muhammad in what the Angel said was "clear Arabic." This has come to mean (for many reasons) that Arabic is the most beautiful language of all. The Qur'an also contains a challenge: if you do not believe that the Qur'an is the word of God, try to make something just as beautiful and good as it! This is the doctrine of inimitability (no copy can be made), or "i'jaz." These two doctrines, along with the inherent difficulty of translation, severely limit the value of translations of the Qur'an. In other words, the only Qur'an is the Arabic Qur'an. English versions of the Qur'an are regarded as commentaries.
Two of the most famous and widely-read English translations of the Qur'an are those by A. Yusuf Ali and M. Marmaduke Pickthall  (and see links below).
Scholars and theologians have identified six main sources for the contents of the Qur'an. The Catholic Encyclopedia states: "The sources of the Koran be reduced to -
- The Old Testament (canonical and apocryphal) and the hybrid Judaism of the late rabbinical schools. During Mohammed's time the Jews were numerous in many parts of Arabia, especially around Medina. Familiarity with them is undoubtedly responsible for many Old Testament stories alluded to in Qur'an. Later Judaism and Rabbinism are equally well represented (Geiger, "Was hat Mohammed aus dem Judenthum aufgenommen?", Wiesbaden, 1833; tr. "Judaism and Islam", Madras, 1898).
- The New Testament (canonical and apocryphal) and various heretical doctrines. On his journeys between Syria, Hijaz, and Yemen, Mohammed had every opportunity to come in close touch with Yemenite, Abyssinian, Ghassanite, and Syrian Christians, especially heretics. Hence, while the influence of orthodox Christianity upon the Qur'an has been slight, apocryphal and heretical Christian legends, on the other hand, are one of the original sources of Koranic faith. (See Muir, op. cit. infra, 66-239; Tisdall, "The Original Sources of the Qur'an", London, 1905, 55-211.)
- Sabaism, a combination of Judaism, Manicheism, and old disfigured Babylonian heathenism.
- Zoroastrianism. On account of Persia's political influence in the north-eastern part of Arabia, it is natural to find Zoroastrian elements in the Qur'an.
- Hanifism, the adherents of which, called Hanifs, must have been considerable in number and influence, as it is known from contemporary Arabian sources that twelve of Mohammed's followers were members of this sect.
- Native ancient and contemporary Arabian heathen beliefs and practices. Wellhausen has collected in his "Reste des arabischen Heidentums" (Berlin, 1897) all that is known of pre-Islamic Arabian heathen belief, traditions, customs, and superstitions, many of which are either alluded to or accepted and incorporated in the Koran. From the various sects and creeds, and Abul-Fida, the well-known historian and geographer of the twelfth century, it is clear that religious beliefs and practices of the Arabs of Mohammed's day form one of the many sources of Islam. From this heathen source Islam derived the practices of polygamy and slavery, which Mohammed sanctioned by adopting them."
Verses of Violence
The Qur'an contains scores of verses exhorting Muslims to engage in violence against non - Muslims:
- "I will cast terror into the hearts of those who disbelieve. Therefore strike off their heads and strike off every fingertip of them" (Sura 8 verse 12). 
- "Say to the Unbelievers, if (now) they desist (from Unbelief), their past would be forgiven them; but if they persist, the punishment of those before them is already (a matter of warning for them). And fight them on until there is no more tumult or oppression, and there prevail justice and faith in Allah altogether and everywhere; but if they cease, verily Allah doth see all that they do" (8:38,39).
- "So when the sacred months have passed away, then slay the idolaters wherever you find them, and take them captives and besiege them and lie in wait for them in every ambush, then if they repent and keep up prayer and pay the poor-rate, leave their way free to them" (Koran 9:5)..
- "The Jews call 'Uzair a son of Allah, and the Christians call Christ the son of Allah. That is a saying from their mouth; (in this) they but imitate what the unbelievers of old used to say. Allah's curse be on them: how they are deluded away from the Truth!" (9:30) .
- "Soon shall We cast terror into the hearts of the Unbelievers, for that they joined companions with Allah, for which He had sent no authority" (Sura 3 verse 151).  This verse commands violence against Christians, who according to Mohammed 'joined Companions with Allah' and are thus infidels who must be killed, subjugated, or converted according to the Koran.
- Koran, by Gabriel Oussani, The Catholic Encyclopedia