From my reading..., the Qur'an was revealed to Muhammad by the angel Gabriel, as occasion required it, over the last twenty-three years of his life. They believe the Qur'an existed eternally in the heaven engraved on a stone tablet.
Muhammed passed on these revelations orally to his Companions, some of whom wrote them down. Others memorised them. Muslim tradition states that the Angel helped Muhammad to collate the revelations periodically, so that at his death there was an exact transcript of what was written on the heavenly tablet.
Modern Scholarship however, has shown there was no complete set of collated and arranged revelations at Muhammad's death. Some time after he died, the revelations were assembled by Muslim Leaders on whatever they had been written down on. This included leaves, stones, camel's shoulder blades etc. and from the memories of those who stored the revelations in their minds. The resulting collections of revelations made in differen places, varied from one another. Caliph Uthman decided to bring order to the situation and had scholars create an official standard text between 650 and 656, which he circulated widely, whilst having other versions destroyed.
The oldest surviving Qur'anic manuscript dates back to the 8th century AD or the second century AH using the Muslim dating system.They are written in a Kufic script showing consonants only. This leaves them open to ambiguity and vowels and other punctuation marks were added later.
Variants continued to exist until the 10th century AD when some Islamic scholars were imprisoned for not destroying their preferred versions. Even until the mid 20th century, two versions of the Qur'an were in use. One in the Muslim world and a second one in use in the North African Muslim world.
The Qur'an does contain contradictions within itself, with earlier revelations changing with later ones. The revelations that are later are considered to "abrogate" the earlier ones (ie replace them). The rationale is that the earlier revelations were appropriate to the early years of Muhammads' mission, but later different conditions necessitated modification of the earlier ones. The resulting difficulty is knowing which is the earlier verse.
It's interesting that the Qur'an does not suggest that Muhammad was to create a new religion, but simply that he should bring people back to the faith of the prophets from Adam onwards. For example, in Sura 2:136 it is written
Say ye, "We believe"
In Allah and the revelation
Given to us, and to Abraham, Ismail, Isaac and Jacob.
And the Tribes, and that given
To Moses and Jesus, and that given
To (all) prophets, from their LORD
We make no difference
Between one and another of them
And we bow to Allah ( in Islam).
Surha 10:94 then recommends consulting the Jews and the Christians "who have been reading the book before thee."
If thou wert in doubt
As to what We have revealed
Unto thee, then ask those
Who have been reading
The Book before thee.
So, the Qur'an according to it's own revelation, was meant to CONFIRM the previous revelations, and Muslims who couldn't understand the teachings were to/are to consult the Christians and Jews who had the Book before them.
Unfortunately, as we see regularly in this Forum, if the Qur'an truly was a continuation of God's revelation given before hand, we would expect to see harmony and continuity. In stead we see contradiction, something which leads to the next point.. .which are the true Revelations from God? The Qur'an says in one breath it is the continuing revelation and that it confirms all that was said in the previous revelations, then it says that Christians and Jews have corrupted their revelations and that the Qur'an is the only only uncorrupted revelation.
As I said earlier in this post, though Muslims try and show where the scribes have made translation errors in the Bible translations, the same is equally true of the Qur'an, a fact not often discussed publically in Muslim circles.