Praise be to Allaah.
The Qur'aan is the word of the Lord of the Worlds, which Allaah revealed to His Messenger Muhammad (peace and blessings of Allaah be upon him), to bring mankind forth from darkness into light:
"It is He Who sends down manifest Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) to His slave (Muhammad) that He may bring you out from darkness into light
[al-Hadeed 57:9 - interpretation of the meaning]
Allaah has told us in the Qur'aan the stories of the earlier and later generations and the creation of the heavens and the earth. He has explained in detail what is halaal and what is haraam, the basics of good manners and morals, the rulings of worship and dealings with others, the lives of the Prophets and the righteous, and the reward and punishment of the believers and disbelievers. He has described Paradise, the abode of the believers, and He has described Hell, the abode of the disbelievers. He has made it (the Qur'aan) an explanation of all things:
"And We have sent down to you the Book (the Qur'aan) as an exposition of everything, a guidance, a mercy, and glad tidings for those who have submitted themselves (to Allaah as Muslims)"
[al-Nahl 16:89 - interpretation of the meaning]
The Qur'aan explains the names and attributes of Allaah and what He has created. It calls us to believe in Allaah, His angels, His Books, His Messengers and the Last Day:
"The Messenger (Muhammad) believes in what has been sent down to him from his Lord, and (so do) the believers. Each one believes in Allaah, His Angels, His Books, and His Messengers. (They say,) 'We make no distinction between one another of His Messengers' - and they say, 'We hear, and we obey. (We seek) Your forgiveness, our Lord, and to You is the return (of all)'"
[al-Baqarah 2:285 - interpretation of the meaning]
The Qur'aan describes the Day of Judgement and what will happen after death - the resurrection, the gathering, the judgment and being brought to account. It describes the Cistern, the Siraat (bridge over Hell), the Balance [in which deeds will be weighed], the blessings and torment, and the gathering of mankind on that great Day:
"Allaah! Laa ilaaha illa Huwa (none has the right to be worshipped but He). Surely, He will gather you together on the Day of Resurrection about which there is no doubt. And who is truer in statement than Allaah?
[al-Nisaa' 4:87 - interpretation of the meaning]
The Qur'aan calls us to examine and ponder the signs of Allaah in the universe and the verses of the Qur'aan:
"Say: 'Behold all that is in the heavens and the earth'"
[Yoonus 10:101 - interpretation of the meaning]
"Do they not then think deeply in the Qur'aan, or are their hearts locked up (from understanding it)?"
[Muhammad 47:24 - interpretation of the meaning]
The Qur'aan is the Book of Allaah for all of mankind:
"Verily, We have sent down to you (O Muhammad) the Book (this Qur'aan) for mankind in truth. So whosoever accepts the guidance, it is only for his own self; and whosoever goes astray, he goes astray only for his (own) loss. And you (O Muhammad) are not a Wakeel (trustee or disposer of affairs, or guardian) over them"[al-Zumar 39:41 - interpretation of the meaning]
The Qur'aan confirms the Books which came before it, the Tawraat (Torah) and Injeel (Gospel), and it is a witness over them, as Allaah says (interpretation of the meaning):
"And We have sent down to you (O Muhammad) the Book (this Qur'aan) in truth, confirming the Scripture that came before it and Muhaymin (trustworthy in highness and a witness) over it (old Scriptures)"[al-Maa'idah 5:48]
After the Qur'aan was revealed, it became the Book for all of mankind until the Hour begins. Whoever does not believe in it is a kaafir who will be punished with torment on the Day of Resurrection, as Allaah says (interpretation of the meaning):
"But those who reject Our Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.), the torment will touch them for their disbelief (and for their belying the Message of Muhammad)"[al-An'aam 6:49]
Because of the greatness of the Qur'aan and the signs, miracles, parables and lessons contained therein, in addition to its eloquence and beautiful style, Allaah says (interpretation of the meaning):
"Had We sent down this Qur'aan on a mountain, you would surely have seen it humbling itself and rent asunder by the fear of Allaah. Such are the parables which We put forward to mankind that they may reflect"[al-Hashr 58:21]
Allaah has challenged mankind and the jinn to produce something like it, even one soorah or one aayah, but they could not do that and will never be able to do that, as Allaah says (interpretation of the meaning):
"Say: 'If the mankind and the jinn were together to produce the like of this Qur'aan, they could not produce the like thereof, even if they helped one another'"[al-Israa' 17:88]
Because the Qur'aan is the greatest of the heavenly Books, the most complete, the most perfect and the last of them, Allaah commanded His Messenger Muhammad (peace and blessings of Allaah be upon him) to convey it to all of mankind, as He says (interpretation of the meaning):
"O Messenger (Muhammad)! Proclaim (the Message) which has been sent down to you from your Lord. And if you do not, then you have not conveyed His Message. Allaah will protect you from mankind" [al-Maa'idah 5:67]
Because of the importance of this book and the ummah's need for it, Allaah has honored us with it. He sent it down to us and has guaranteed to preserve it. Allaah says (interpretation of the meaning):
"Verily, We, it is We Who have sent down the Dhikr (i.e. the Qur'aan) and surely, We will guard it (from corruption)" [al-Hijr 15:9]
Source: From Usool al-Deen al-Islami by Shaykh Muhammad ibn Ibraaheem al-Tuwayjri
Answer is also located at Islam Q & A (http://www.islam-qa.com/en/ref/10197/)