The Holy Qur'an was revealed to Prophet Mohammed (Peace Be Upon Him) . Its the word of Almighty ALLAH and the base of ISLAM. One who accepts it is within the boundaries of Islam and the one who rejects is amongst the disbelievers.
At the age of 40 , Muhammed(Peace Be Upon Him) was entrusted and crowned with Prophethood and it was at this time that the first Verse was revealed . Most of the religious scholars are united in their histrionics that it was 21st Day of month of Ramazan i.e. 10th August 610 AC (After Christ - Muslims should not say AD , which falsely means After Death) . Thereafter the verses of Qur'an were revealed regularly on occassions whenever needed . The Holy Qur'an wasn't revealed as a complete Book at one instant like Injeel on Isa (Christ) PBUH , Tauraat on Musa(Moses) PBUH or Zaboor on Dawood(David) PBUH . The verses of Qur'an were revealed one at a time depending upon the need for the orders . Sometimes one , sometimes two , at times only half of the verse or at times a complete Surah (Chapter) was revealed . This lasted for 23 Years till Prophet Mohammed (Peace Be Upon Him) passed away . The revealation of verses is called as "Wahi" . There are several ways in which the Wahi (Verses revealed) had come to Prophet Mohammed (Peace Be Upon Him) :
1) The Angel Jibrael used to come with the revealations and Prophet Mohammed (Peace Be Upon Him) could hear the sound of a gentle bell . This was the toughest of all the other forms and Prophet Mohammed (Peace Be Upon Him) said that I felt as if my soul is coming out of the body.
2) The Angel Jibrael puts the verses into Prophet Mohammed's (Peace Be Upon Him) heart .
3) The Angel Jibrael comes in form of man and reads the verses to Prophet Mohammed (Peace Be Upon Him) , which Prophet Mohammed (Peace Be Upon Him) used to recite and memorize
4) ALLAH directly speaks to Prophet Mohammed (Peace Be Upon Him) , as he did in the night of Meh'raj (the night when Prophet Mohammed (Peace Be Upon Him) , went to the Heavens to meet the Almighty ALLAH) .
Certain verses of Qur'an which were applicable only for a particular moment of time , are called Man'sookh (No longer applicable)
When Prophet Mohammed (Peace Be Upon Him) had passed away , by this time most of the Sahaa'baaz (Companions of Prophet Mohammed (Peace Be Upon Him) ) had memorised the Holy Qur'an by heart like the way Prophet Mohammed (Peace Be Upon Him) used to recite. Every year during the month of Ramazan Prophet Mohammed (Peace Be Upon Him) used to do Daur (recite the verses memorised) in front of Jibrael , the Angel . At times it used to be the other way too , that Angel Jibrael used to recite for Prophet Mohammed (Peace Be Upon Him) . It was during the last Ramazan , the year in which Prophet Mohammed (Peace Be Upon Him) passed away , that Prophet Mohammed (Peace Be Upon Him) did the Daur (recite the verses memorised) of the whole Qur'an twice . Hazrat Zaid Bin Saabit (Razi-Allahhu-Anhu -RA) , a companion of the Prophet Mohammed (Peace Be Upon Him) , was present on both times throughout the Daur and he said that no verses after that were declared Man'sookh (not applicable) . So the whole Qur'an was complete at that time and the Companions memorised that in the same way as was recited by Prophet Mohammed (Peace Be Upon Him) .
During the Khilafat of Hazrat Abu-Bakr Siddiq (RA) , the first Khalifa , most of the Companions of Prophet Mohammed (Peace Be Upon Him) who had memorised the Qur'an by heart were getting martyred in Jihad . So Hazrat Umar Bin Khattab (RA) suggested Hazrat Abu Bakr of the need to compile the whole Qur'an on paper . On the consent of the Khalifa, Hazrat Zaid Bin Saabit (who used to be present during the revealations and during the Daur of Qur'an by Prophet Mohammed (Peace Be Upon Him) ) was instructed to compile the Holy Qur'an on the paper as was recited by Prophet Mohammed (Peace Be Upon Him).
Hazrat Umar (RA) , during his Khilafat made it a point to propagate and encourage people to memorize the Qur'an .
Hazrat Usman (RA) during his Khilafat , identified the different ways (accent) in which the verses were read and they are SEVEN different ways . These were sent to the empires and they were instructed to implement it so that no confrontations arise out of it.
And today we have a religious book , THE HOLY QUR'AN , nobody can present an example like it , not even the likes of a single verse as ALLAH says
"And if you (Arab pagans, Jews, and Christians) are in doubt concerning that which We have sent down (i.e. the Qur'an) to our slave Muhammed (Peace Be Upon Him), then produce a Surah (chapter) of the like thereofand call your witnesses (supporters and helpers) besides ALLAH if you are truthful. " (2:23)
" Or do they say: "He Muhammed (Peace Be Upon Him) has forged it?" Say: "Bring then a Surah (chapter) like unto it, and call upon whomsoever you can besides ALLAH, if you are truthful!" (10:38)
Now 14 centuries years since then , not a single verse is changed and till Qayamet , it will be protected Inshallah, no power can change it . ALLAH (Subhana-wa- Ta'alaa) says
"We have , without doubt ,
Sent down the message ;
And We will assuredly
Guard it (from corruption) "