The Qur'an is one of the two sources which form the basis of Islam. The second source is the Sunnah of the Prophet (saw). What makes the Qur'an different from the Sunnah is primarily its form. Unlike the Sunnah, the Qur'an is literally the Word of Allah (swt), whereas the Sunnah was inspired by Allah but the wording and actions are the Prophet's (saw). The Qur'an has not been expressed using any human's words. Its wording is letter for letter fixed by no one but Allah.
Prophet Muhammad (saw) was the final Messenger of Allah to humanity, and therefore the Qur'an is the last Message which Allah (swt) has sent to us. Its predecessors such as the Torah, Psalms, and Gospels have all been superseded. It is an obligation - and blessing - for all who hear of the Qur'an and Islam to investigate it and evaluate it for themselves. Allah (swt) 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 (saw) 1400 years ago.