Preservation of The Quran.

In this Book, the Holy Prophet's life, the history of the Arabs and the events which occurred during the period of the revelation of the Quran have not been mingled with the Divine Verses, as is the case with the Bible. The Quran is the pure word of God. Not one word therein is not divine. Not a single word has been deleted from its text. The Book has been handed down to our age in its complete and original form since the time of Prophet Muhammad. From the time the Book began to be revealed, the Holy Prophet had dictated its text to the scribes. Whenever some Divine Message was revealed, the Holy Prophet would call a scribe and dictate its words to him. The written text was then read out to the Holy Prophet, who, having satisfied himself that the scribe has committed no error of recording, would put the manuscript in safe custody. The Holy Prophet used to instruct the scribe about the sequence in which a revealed message was to be placed in a particular Surah (chapter). In this manner, the Holy Prophet continued to arrange the text of the Quran in systematic order till the end of the chain of revelations. Again, it was ordained from the beginning of Islam that a recitation of the Holy Quran must be an integral part of worship. Hence the illustrious Companions would commit the Divine verses to memory as soon as they were revealed. Many of them learned the whole text and a far larger number had memorized different portions of it.

Method of preservations of the Quran during the Prophet's time
Besides, those of the Companions (pbut) who were literate used to keep a written record of several portions of the Holy Qur'an. In this manner, the text of the Holy Qur'an had been preserved in four different ways during the lifetime of the Holy Prophet (PBUH):
a) The Holy Prophet (PBUH) had the whole text of the Divine Messages from the beginning to the end committed to writing by the scribes of revelations.
b) Many of the Companions learned the whole text of the Qur'an, every syllable of it, by heart.
c) All the illustrious Companions, without an exception, had memorized at least some portions of the Holy Qur'an, for the simple reason that it was obligatory for them to recite it during worship. An estimate of the number of the illustrious Companions may be obtained from the fact that one hundred and forty thousands Companions had participated in the Last Pilgrimage performed by the Holy Prophet (PBUH).
d) A considerable number of the literate Companions kept a private record of the text of the Qur'an and satisfied themselves as to the purity of their record by reading it out to the Holy Prophet (PBUH).

Method of preservations of the Quran after the demise of the Prophet:
It is an incontrovertible historical truth that the text of the Holy Qur'an extant today is, syllable for syllable, exactly the same as the Holy Prophet (PBUH) had offered to the world as the Word of God. After the demise of the Holy Prophet, the first Caliph Hadhrat Abu Bakr (PBUH) assembled all the Huffaz and the written records of the Holy Qur'an and with their help had the whole text written in Book form. In the time of Hadhrat 'Uthman (PBUH) copies of this original version were made and officially dispatched to the Capitals of the Islamic World. Two Of these copies exist in the world today, one in Istanbul and the other in Tashkent. Whosoever is so inclined may compare any printed text of the Holy Qur'an with those two copies, he shall find no variation.
And how can one expect any discrepancy, when there have existed several million Huffaz in every generation since the time of the Holy Prophet (PBUH) and in our own time? Should anyone alter a syllable of the original text of the Qur'an, these Huffaz would at once expose the mistake.
In the last century, an Institute of Munich University in Germany collected FORTY-TWO THOUSAND copies of the Holy Qur'an including manuscripts and printed texts produced in each period in the various parts of the Islamic World. Research work was carried out on these texts for half a century, at the end of which the researchers concluded that apart from copying mistakes, there was no discrepancy in the text of these forty-two thousand copies, even though they belonged to the period between the 1st Century Hijra to 14th Century Hijra and had been procured from all parts of the world. This Institute, alas! perished in the bombing attacks on Germany during World War II, but the findings of its research project survived.
Another point that must be kept in view is that the word in which the Qur'an was revealed is a living language in our own time. It is still current as the mother tongue of about a hundred million people from Iraq to Morocco. In the non-Arab world too, hundreds of thousands of people study and teach this language.
The grammar of the Arabic language, its lexicon, its phonetic system and its phraseology, have remained intact for fourteen hundred years.
A modern Arabic-speaking person can comprehend the Holy Qur'an with as much proficiency as did the Arabs of fourteen centuries ago. This, then, is an important attribute of Muhammad (PBUH), which is shared by no other Prophet or Leader of Religion. The Book which God revealed to Him for the guidance of mankind is today's in its original language without the slightest alteration in its vocabulary.
From Islamic Web site,