The Qur'an is a Message from God to humanity. It was transmitted to us in a chain starting from the Almighty Himself to the angel Gabriel to the Prophet Muhammad. This message was given to the Prophet in pieces over a period spanning approximately 23 years (610 CE to 622 CE). The Prophet was 40 years old when the Qur'an began to be revealed to him, and he was 63 when the revelation was completed.
The Qur'an is one leg of two which form the basis of Islam. The second leg is the Sunnah of the Prophet . What makes the Qur'an different from the Sunnah is primarily its form. Unlike the Sunnah, the Qur'an is quite literally the Word of God, whereas the Sunnah was inspired by God but the wording and actions are the Prophet's.
Prophet Muhammad was the final Messenger of God to humanity, and therefore the Qur'an is the last Message which God has sent to us. Its predecessors such as the Torah, Psalms, and Gospels have all been superceded. God has guaranteed that He will protect the Qur'an from human tampering, and today's readers can find exact copies of it all over the world. The Qur'an of today is the same as the Qur'an revealed to Muhammad.