The Quran is God revelations to Prophet Mohamed (peace upon him). These revelations of Quran by God to Prophet Mohamed was through the Angel Gabriel (Jibril) and started in year 610 AD when prophet Mohamed was in Mecca (Makkah) and was by then 40 years old. The revelations continued while Prophet was in Both Mecca and Medina (currently in Saudi Arabia) since then and was completed by year 632 AD (year of death of the prophet).
The long period for full revelation of the Quran (around 22 years) allowed the Quran to be fully memorized and documented by many Muslims. It was recited in the mosque by prophet Mohamed during the daily five prayers without single letter change. Many Muslims since then until now are memorizing Quran fully without single letter difference. This explains why Quran remains by God will, and will remain by God will, without single letter change.
Most of authors in the introductory pages of their books apologize for any possible mistake in their work even if it's only a compliment. However, Allah & Allah alone as He is the one who does know everything about everything, challenges to find any discrepancy in His holy book Quran. Quran says:
أَفَلَا يَتَدَبَّرُونَ الْقُرْآنَ ۚ وَلَوْ كَانَ مِنْ عِندِ غَيْرِ اللَّـهِ لَوَجَدُوا فِيهِ اخْتِلَافًا كَثِيرًا ﴿٨٢﴾
Meaning English translation:
" Then do they not reflect upon the Qur'an? If it had been from [any] other than Allah, they would have found within it much contradiction. (82)" [Quran, Chapter 4, verse 82]
Refer to related question on what is revealed in Quran.
Note:
- Name Mohamed may be spelled Mohamad, Muhammad, Muhammed, Mohammad, or Mohammed.
- Quran may be spelled Qura'n, Quraan, Kuran, Kura'n, Kuraan, Koran, Kora'n, or Koraan.
The Holy Quran was revealed to Holy Prophet Muhammad (Peace be Upon Him) by Allah (God) through the Angel Gabriel as the final message of Allah (God) to mankind.
The Holy Quran was written to create a permanent copy of Allah's (God's) final message to mankind after this there would be no more messages from Allah (God). Due the sheer importance of this message, when ever a part of it was revealed to Holy Prophet Muhammad (Peach be Upon Him) it was immediately written almost every time by at least 3 followers of Prophet Muhammad (Peace to be upon him) to create a hard copy in the presence of Holy Prophet (Peace be Upon Him). It was also immediately memorized by those people. Thus eliminating the chances of it being corrupted by anything and ensuring that what originally was revealed by Angel Gabriel is what exactly is in Quran with no single letter change.