Muslims believe in all the Books that Allah (God) gave to His various Rusul (Messengers) at different times for guidance of mankind.

Some of those Rusul who were given Books by Allah (God) are mentioned in the Holy Qur-an.
Ibrahim A (Abraham) was given Suhuf Ibrahim, Musa A (Moses) was given the Torat (Torah), Dawud A (David) was given the Zabur (Pslams), and 'Isa A (Jesus) was given the Injil (Gospel). Besides these Books, Allah (God) has sent Guidance to all nations and people through His Rusul (Messengers).

With time, people altered, modified, changed, falsified and edited those Books for their own vested interests.
Everytime that happened, Allah (God) sent another Book through another Rasul (Messenger) for Guidance of his people. It continued till Allah (God) sent Muhammad S as His last Rasul (Messenger) with His final Message - the Holy Qur-an, as guidance for the entire mankind. Since no other Message will now be sent, Allah (God) guarenteed the preservation of the Holy Qur-an.

The Holy Qur-an is the Word of Allah (God). It was revealed to the last Messenger of Allah (God), Muhammad S in Arabic language. Although it is Guidance to the entire mankind, only sincere and God fearing people are able to benefit from its Guidance.

Qur-an is the final version of the same Message that Allah (God) had sent earlier to mankind through many of His Rusul (Messengers).

The Holy Qur-an is the source of all Islamic religious beliefs and practices. It is the most read and the most quoted book in the world. This is the only Book, which has been miraculously memorized completely by millions of people by heart since its revelation, who do not even know the language. 

Qur-an was revealed in small portions over a period of 23 years. There are 114 Chapters in Qur-an. Each chapter of Qur-an is called a Surah. Each Surah is divided into various Ayat (Verses). Some Suwar contain more verses while others less. Surah al-Baqarah, the second chapter of Qur-an, is the biggest of all. It contains 286 Ayat (Verses). The shortest Surah of Qur-an is Surah al-Kauthar (A river in Paradise). It contains only three Ayat (Verses). Though there are other Suwar that contain only three Ayat (Verses) but they contain more words and lengthy sentences. The other Suwar which contain only three Ayat (Verses) are Surah al-‘Asr (The Time) and Surah an-Nasr (The Help).
(Suwar is the plural of Surah and Ayat is the plural of Ayah).

Qur-an is also divided into thirty almost equal portions. Each of these portions is called a Juz in Arabic.

The Qur-an was revealed to the Messenger of Allah, Muhammad S in small portions. Some portions of the Qur-an were revealed in Makkah while other portions of it were revealed in Madinah. The Suwar that were revealed in Makkah, are known as Makki Suwar. The Suwar revealed in Madinah, are known as Madani Suwar.  

The Makkan period of revelation was from 610 AD to 622 AD. The Madani period was from 622 AD to 632 AD. Eighty-six (86) Suwar were revealed in Makkah, and twenty-eight (28) Suwar were revealed in Madinah.

Makki Suwar contain 4417 Ayat (Verses), Madani Suwar contain 1819 Ayat (Verses).
The total number of Ayat (Verses) in the Hoy Qur-an is six thousand two hundred thirty six (6,236).

The Messenger of Allah, Muhammad S received the first Wahi (Message from Allah) in 610 AD, in the small cave in the mountain of Hira. That mountain is now known as Jabl Nur (Mountain of Radiance). It is located about two and a half miles (Approximately Four Kilometers) away from the Ka’bah in the city of Makkah.

In the first Wahi (Message from Allah) Muhammad S received the first five Ayat (Verses) of Surah (Chapter) al-‘Alaq (The Clot).

Surah Al-Fatihah (The Opening Chapter) was the first complete chapter to be revealed and Surah an-Nasr was the last.

When the Qur-an was being revealed, Muhammad S used to recite the revealed verses to his Sahabah (Companions). They used to learn these verses by heart. Some of the companions used to write them on palm leaves, animal hides and stone tablets etc. The Suwar (Chapters) and Ayat (Verses) of Qur-an are not arranged in their chronological order of revelation. Muhammad S used to set the place of various Suwar (Chapters) and Ayat (Verses) himself.

Although the entire Qur-an was committed to writing including the sequence of Suwar (Chapters) and Ayat (Verses) during the lifetime of Muhammad S, it was not compiled in a single volume in the form of a book. Its various Suwar (Chapters) and Ayat (Verses) were written was available with various Sahabah (Companions) on different writing mediums of that time. 

There were several designated Katib Al-Wahi (Writers of Messages from Allah), who used to write down the divine revelations whenever Muhammad S narrated to them after receiving the Wahi (Message from Allah).

After the demise of the Muhammad S, many newly converted tribes revolted. Most of them decided to follow most principles of Islam, except the institution of Zakah (Poor Due Charity). One cannot pick and choose beliefs in Islam. There is no compromise with the basic Islamic beliefs.
Abu Bakr R became the first Khalifah (Caliph) of Muslims. Soon after assuming this responsibility, he had to face severe challenges in the form of several revolts. He also had to face challenges from several claims of false Prophethood from several apostates. He decided to firmly deal with them. He sent Muslim armies to suppress the rebellion. As a result, many of those Huffaz (Those who had memorized Qur-an by heart) died.
‘Umar bin Khattab R, another prominent Sahabi (Companion of Muhammad S) pondered upon the situation. He felt an urgent need to compile the scattered portions of Qur-an into one volume. He suggested it to Abu Bakr R, who was initially reluctant to undertake such a task. His reasoning was that since Muhammad S did not make any such compilation during his lifetime, so how could he? But after thinking over the matter, he agreed to ‘Umar bin Khattab R’s suggestion. Subsequently, a committee was set up for compiling the Qur-an. It was headed by Zayd bin Thabit R, the secretary of Muhammad S, who also was the main Katib Wahi (Writer of Messages from Allah). This team compiled the Holy Qur-an in one volume. This copy of Qur-an was verified by all Sahabah (Companions of Muhammad S) who used to memorize each Ayah and Surah of the Qur-an as it was revealed to Muhammad S.  

This copy of Qur-an was kept in the custody of Hafsah bint ‘Umar R, one of the holy wives of Muhammad S. After Abu Bakr R, ‘Umar bin Khattab R became the second Khalifah (Caliph) of Muslims, but he, for some reason, did not publish the compiled version of the Qur-an.
It was during the tenure of the third Khalifah (Caliph) of Muslims, ‘Uthman bin ‘Affan R, a standardized version of Qur-an was made from the available document and its copies were sent to all other towns and cities. The Qur-an which we read today is the exact copy of the one published by the third Khalifah (Caliph) of Muslims, ‘Uthman bin ‘Affan R.

There are three main themes of Qur-an, Wahdah (Oneness of Allah), Risalah (Allah‘s Messengers and the finality of the Messengers) and Akhirah (Day of Judgment and life after death).

The main emphasis of Qur-an is Tawhid i.e. the Oneness of God, and the finality of the Messengership of Allah, Muhammad S. Besides, it contains the stories of previous Messengers, moral teachings and the injunctions of Shari’ah. It also informs us about other revealed books, about angels, about the Last Day of Judgment and Paradise and Hell.  

The Qur-an is the most comprehensive book of law. It deals with each and every aspect of human life. Its rules are easy and simple. It does not unnecessarily stress mankind. It does not lead to asceticism. It clearly states:

The month of Ramadan in which was revealed the Qur-an, a guidance for mankind and clear proofs for the guidance and the criterion (between right and wrong). So whoever of you sights (the crescent on the first night of) the month (of Ramadan i.e. is present at his home), he must observe Saum (fasts) that month, and whoever is ill or on a journey, the same number [of days which one did not observe Saum (fasts) must be made up] from other days. Allah intends for you ease, and He does not want to make things difficult for you. (He wants that you) must complete the same number (of days), and that you must magnify Allah [i.e. to say Takbir (Allahu-Akbar; Allah is the Most Great) for having guided you so that you may be grateful to Him. Surah (Chapter) Al-Baqarah (The Cow)The Qur-an 2:185