Essential to learning the history of the Quran is the study of the stages of its revelation, which is usually referred to by the term "'Tanzeel' of the Quran", literally, the descent or the 'sending down' of the Quran from the Heavens to the Earth. Some of the aspects to be discussed in relationship to this would be the meaning of the 'Tanzeel' and why it happened in stages, what was revealed first and what was revealed last, and the significance of understanding the sequence in revealing the different parts of the Quran, among others.
Certainly, the study of the 'Tanzeel' of the Quran is one of the most important aspects of the Sciences of the Quran, because it is related to the beginning of the last revelation, and to being certain of its descent from Allah, which is the foundation of believing in the Quran as the Word of Allah that came from Him, as well as in the truthfulness of the Message of Prophet Muhammad  sallallaahu  `alayhi  wa  sallam ( may  Allah exalt his mention ).
The Meaning of 'Tanzeel'
Lexically, the term 'Tanzeel' is derived from the Arabic root 'Nuzool', which means the descent or the movement from a high place down to a lower one. This is exactly what happened in the process of revelation where the Angel Jibreel (Gabriel) used to come down from the Heavens with a varying number of verses of the Quran to deliver to Prophet Muhammad  sallallaahu  `alayhi  wa  sallam ( may  Allah exalt his mention ). The exact nature of this 'Nuzool' and how Jibreel actually brought the Quran with him are matters of the unseen, the details of which Allah chose not to clarify for us, perhaps because they involve matters that may be very difficult for humans to imagine or grasp. What we should understand is that this 'Nuzool' happened in a fashion that is befitting to the greatness and sacredness of the Word of Allah. Allah Almighty Says about the Holy Quran (what means): “ (It is) a Quran which We have divided (into parts from time to time), in order that thou (O Muhammad) might recite it to people at intervals: We have revealed it by stages (in 23 years).” [Quran 17:106]
The Quran was revealed in two stages:
Stage One:
In this stage, the Quran was brought by Jibreel from the Preserved Tablet (in the Seventh Heaven) down to the First Heaven (sky) and placed in the 'Place of Power or Honor'. First, the Quran mentions the Preserved Tablets in the verse (which means):  “Nay! This is a Glorious Quran, inscribed in a Preserved Tablet” [Quran 85:21-22]
Then it tells of the fact that it was sent down during a specific night, in the verse (which means): “By the Book that makes things clear; - We sent it down during a Blessed Night: for We (ever) wish to warn (against Evil). In the (Night) is made distinct every affair of wisdom.” [Quran 44:2-4]
And (which means): “We have indeed sent down this during the Night of Power.” [Quran 97:1].
These two verses indicate that the Quran was revealed in a single blessed night in the month of Ramadan that is called the 'Night of Power, or Decree'.
Stage Two:
This stage of revelation lasted for about 23 years in which the Quran was sent down in response to the development of events and in accordance with the sequence needed to complete the delivery of the Message.
The Prophet  sallallaahu  `alayhi  wa  sallam ( may  Allah exalt his mention ) after receiving revelation, would automatically memorize the Quranic verses that were revealed to him and would recite them to whomever was in his company, and order them (the Companions may Allah be pleased with them) to write them down immediately. The Messenger  sallallaahu  `alayhi  wa  sallam ( may  Allah exalt his mention ) himself used to keep a copy of the revealed portions in his house. The Quran, the last of Allah’s Scriptures, is divided into 114 'Surahs' (Chapters) of unequal length. It is the first fundamental source of Islamic Sharee’ah (Islamic Law). Allah, Most High, Says (what means): “… A Quran which We have given seriatim (separated out) in order that you may recite it to the people at intervals and We have caused it to come down successively (by stages).” [Quran 17:106]
But verse 6:7 observes (what means): “If We had sent unto you (O Muhammad) a written (message) on parchment, so that they could touch it with their hands, the unbelievers would have been sure to say: “This is nothing but obvious magic.’” [Quran 6:7]
Recording cynical speeches of the unbelievers, Almighty Allah Says (what means): “Why has the Quran not been vouchsafed to him (Muhammad) in one single whole?” [Quran 25:32]
nd the answer to that is the verse (which means): “…that your heart (O Muhammad) may be strengthened, We have rehearsed it to you in slow, well-arranged stages, gradually.” [Quran 25:32]
There is no nation that has ever cared about, revered, and preserved its Divine Scripture as the Muslim nation has cared about, revered and preserved the Quran. Unlike the other Divine Scriptures, the Quran is not kept in the hands of a particular group or elite of the Muslims, and for this reason, it is not subject to suspicion that it might have been tampered with or altered. Rather, it has always been within the reach of all Muslims.

For a Muslim, the Quran is the word of God; literal word of God communicated to His last Prophet and Messenger, Prophet Muhammad (peace be upon him), through Angel Gabriel.

What is The Quran and Why Should I Believe It?
For a Muslim, this is the ultimate source of authority because it’s not only revelation or inspiration of God, it’s verbatim word or revelation of God.

Now as we compare that so that our respected audience from the Jewish or Christian background can at least relate to this understanding, this is not unusual.

In fact, In Islam, Judaism, and Christianity there is this notion of God revealing His will to humankind. I believe that there are other religions as well who depend as authority on scriptures also that they believe have some form of revealed nature.

Comparing the Quran and Bible
If we look into the Bible, we will find that there are instances or statements that appear to be also sort of verbatim word of God, or the closest thing to that.

For example, the Ten Commandments when God said to Moses “Thou shall do this, or Thou shall not do this”, it doesn’t come in a form where God says I inspired Moses or Moses says God revealed this to me. It appeared to be verbatim word of God. So that comes the closest to the concept of revelation of the Quran to Muslims.

The only difference perhaps I can see is that in the Bible, there are like I said sections which appear to have been direct word of God, not just inspiration, but the Bible does contain also biography about prophets, writings by their followers, reports of their particular religious experiences, so these are all intermingling.

Many of my Christian and Jewish brothers and sisters, when we get into dialogue you hear them also saying that from their perspective also there is another from of revelation which is more of an inspiration.

6 Proofs that Muhammad Couldn't Have Authored the Quran
You read in the Hebrew scripture God inspired His servant so and so. So it implies that the revelation is not exactly word for word what God revealed, like what could be the case with the Ten Commandments.

But mainly it’s more of an inspiration, and in Islamic tradition we have a parallel to that also and that is in the so-called Hadith, or the words and actions of the Prophet of Islam, because Hadith or the sayings of the Prophet are believed by Muslims also to be inspired by God, but it is not exactly word for word because the Prophet used his own words to express or communicate those messages. That might be to some extent parallel to what can be found in the Bible.

So in that sense it does not appear to me really that Muslims’ understanding about the Quran is that vastly different from the basic notion of revelation, especially in revealed religions of the People of the Book, as the Quran called them, in that sense and in terms of its authority and what it means to Muslims.

Muslims perhaps are more confident about the revelatory nature of the Quran as word for word revelation. This is a basic article of faith in Islam without which a Muslim cannot claim really to be a Muslim.

So we can’t compare apples and oranges when you get some kinds of scriptures for example that may combine the word of God and interpretation of humans, it would not from the Muslim perspective have the same authority as a book like the Quran which is A to Z nothing but the verbatim word of God.

The overall message of the Quran is the same overall message that God revealed to all of the prophets from Adam down to the last Prophet Muhammad (peace be upon him), and the Quran seems to affirm that.

If I were to summarize it in a succulent manner, the topic of the Quran is the human and his or her relationship with the Creator, and also create the relationship with other creatures of God, whether humans or others. That’s the topic and the focus of the Quran.

Has the Quran Ever Been Changed?
The Quran’s Core Message
In essence, the message of the Quran is to bring to our attention that we were not created in vain, and that we will be held responsible for our conduct on this earth.

In the Quran, God indicates that He created the human to be His trustee on earth. This is actually an honor given by God to the human being, dignity to be the trustee of God even though He doesn’t need our worship and our obedience. It is for our own benefit.

The message of the Quran is to say or believe that guidance that is coming from the Creator of the universe is the ultimate guidance, for God has the ultimate wisdom, power, and knowledge.

And God alone is the ultimate authority and should be obeyed without qualification, obedience that is based not just on fear or reward but on the love of God.

It indicates again that one should look forward, like you mentioned for example in terms of daily life, one should look after the Quran for guidance.

The Quran generally gives broader guidance in most aspects within concepts, and there are some details there, but normally to the broader guidance so as to conduct our lives according to the will of God.

And the Quran finally indicates that since a human being is a free agent, he or she can choose to obey God or disobey, to believe or reject faith.

Then that kind of privilege has a responsibility that goes with it because ultimately we have a responsibility to return back to God. There will be resurrection. There will be life after death in which people would be rewarded, or otherwise, according to their conduct.

So that in a way can be put as a capsule or a capsulized nature or summary on the Quran, because it is in my humble belief as a Muslim that all prophets of God have taught this exact same core message as the Quran taught.