The Arabic term ‘wahi’ when used in Islamic terminology literally means an ‘inspiration’ which Allah puts or downloads directly into someone’s heart. The Messenger of Allah (saws) received ‘wahi’ or inspirations of two types and they are termed in Islamic Jurisprudence as:
The complete Qur’an was revealed through the ‘Wahi Jalee’….Words of Allah which the Prophet (saws) distinctly recognized and was under the Precise Command of his Lord to relay and declare it verbatim as it was received unto all of mankind as the official ‘Message of Allah.
Other than the Precise and Distinct Message of the Glorious Quran, the Lord Most High sent many a message or inspiration directly to the Prophet’s (saws) heart through the ‘Wahi Khafi’….like what to do, what not to do, how to do, some news of the Hereafter, etc. Although the ‘Wahi Khafi’ were also inspirations which Allah sent directly to the Prophet’s (saws) heart, the Messenger of Allah (saws) was guided to and able to clearly distinguish between the Revelations of the Quran (Wahi Jaleel) and the hidden inspirations (Wahi Khafi). 
The word wahy has been used in the Qur’an to express different meanings, for instance it is used to put some message into the heart: We infused this into the (mind of) Moses’s mother. Qur’an.Sura h Qasas28:7
Assignment of duty to both living and non-living things: And your Lord commanded the Bee to build its cells in the hills. Qur’an.Sur ah Nahl 16:68
And He assigned to each heaven its duty and command. Qur’an.Sur ah Fusilath or Ha meem 41:12
But the word wahy has been used more often in the Qur’an for the divine revelation made to the prophets:
“Surely we have revealed (awhaina) to thee as we revealed to Noah and the Prophet after him........”. Qur’an.Sur ah Nisa 4:163
Then we revealed to Moses to strike the Sea with his staff. Qur’an.Sur ah Shu’ara 26:63
The main purpose of revelation are given below which is derive from the teachings of Qur’an and Sunnah.
“And your god is one God. There is no god but He, the Most Gracious, the Most Merciful.” Qur’an Surah, 2:163. Say (O Muhammad (May Allah honor Him and grant Him peace)): "He is Allah, (the) One.” Qur’an.Sur ah Ikhlas 112:1
The most important topic mentioned throughout the Qur’an is the belief in the One, True God. Allah (Glory be to Him) informed us that He has no partner, no son, no equal, and that none has the right to be worshipped except Him alone. Nothing is comparable to Allah (Glory be to Him) and none of His creation resembles Him. The Qur’an also rejects the notion of attributing human qualities and limitations to Allah (Glory be to Him).
“And worship Allah and associate none with Him.” Qur’an.Sura h Nisa 4:36.
Since Allah (Glory be to Him) alone is the only One worthy of worship, false deities and false gods must be rejected. The Qur’an also rejects the notion of attributing divine qualities to anyone or anything other than Allah (Glory be to Him).
The basic reason for revelation is to guide people. It’s a human nature to get misguided easily as Shaytan is an open enemy for all the human beings, Qur’an.Sur ah Baqrah 2:208. So Allah sent revelations to guide these people to save them from misguidance. For example, Allah (Glory be to Him) said in Qur’an “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).” Qur’an.Surah Baqrah 2:185. Now Qur’an is the final revelation and there will not be any more revelation after this because Qur’an is the only book for which Allah (Glory be to Him) took the responsibility for protecting it from corruption.Qur’an.Sura h Hijr 15:9.
The Qur’an contains many narratives with beneficial lessons, including the true stories of previous Prophets such as, Adam, Noah, Abraham, Jesus and Moses (peace be upon them). Of these stories, Allah says, “Indeed in their stories, there is a lesson for men of understanding.” Qur’an.Su rah Yusuf 12:111.
This Noble Book reminds us that everyone will taste death and will be held accountable for all their actions and sayings: “We shall set up scales of justice for the Day of Judgment, so that not a soul will be dealt with unjustly in the least…” Qur’an.Sur ah Anbiya 21:47.
Importantly, the Qur’an teaches that the purpose of life is to worship God alone, and live one’s life according to the way of life prescribed by Him. In Islam, worship is a comprehensive term that includes all actions and sayings (whether private or public) that Allah (Glory be to Him) loves and is pleased with. Therefore, by doing what Allah Allah (Glory be to Him) commands, a Muslim is worshipping Allah (Glory be to Him)and fulfilling his purpose in life. 
There were many books that were revealed to the Prophets. Qur’an.Sura h Ma’idah 5:66. By name there are four revelation mentioned in the Qur’an. Taurat, revealed to Musa (Moses) Qur’an.Sura h Baqarah 2:53; Zabur, revealed to Dawood (David) Qur’an.Sur ah Nisa 4:163, Qur’an.Sur ah Isra 17:55; Injeel, revealed to Isa (Jesus) Quran.Surah Ma’idah 5:46, and Qur’an revealed to Muhammad (peace be upon them all) Qur’an.Sura h An’am 6:19.
Ibn Al-Qayyim said:
The Divine Revelation was the start of a new era in the history of humankind when it descended to the Prophet, after it had ceased, and humankind was straying in darkness.
The descent of the Divine Revelation was difficult for the Messenger, as is evident in the aforementioned text, although the Messenger, was the most courageous person ever and had the bravest of hearts as indicated by incidents throughout the next twenty-three years. The revelation was not man addressing man. It was an address by an angel bearing the words of Allaah The Almighty to the Prophet, whom Allaah selected to bear His message and communicate it to humanity.
It was an extremely grave situation and a great responsibility that was manageable by only the person whom Allaah The Almighty chose to bear and communicate the Divine message.
The seriousness of the situation was depicted by the part of the Hadeethwherein the Prophet, said, to his wife, Khadeejah “I fear for myself.”It is also explained by what ‘Aa’ishah said, “The Messenger went back with his heart pounding violently, until he came to Khadeejah bint Khuwaylid and said: ‘Cover me! Cover me!’ His family did so, until his fear subsided.”