The topic of tafseer is the most important topic of 'uloom al-Quraan, since in many ways it is the primary goal of 'uloom al-Quraan — to understand and implement the Qur'aan properly. This has also been the first topic of 'uloom al-Quraan to have been written on, and without a doubt the one in which most of the works in this field have been written about.
I. The Definition of Tafseer and Ta'weel
The word 'tafseer comes from fassara', which means, 'to explain, to expound, to elucidate, to interpret.' The word tafseer is the verbal noun of fassara', and means 'the explanation or interpretation of something.'
According to another opinion, the word tafseer is a transposition from s-f-r, which means, 'to expose, to uncover.' Thus, a woman who uncovers her face is called a 'saafirah', and the act of uncovering her face is called 'sufoor.' Therefore, according to this definition, 'tafseer would mean uncovering the meanings and exposing the secrets of the Qur'aan. However, the stronger opinion is the first one, even though both of these meanings are correct.
In Islaamic sciences, tafseer is defined to be: The science by which the Qur'aan is understood, its meanings explained, and its rulings derived.
Another common word that is heard in this subject is the word 'taweel'. What, then, is the difference, if any, between tafseer and taweel?
The word 'ta'weel' is from a-w-l, which means 'to return, to revert,' which implies going back to the original meaning of a word to see what its meanings and connotations are. The meanings of the word 'ta'weel' were given earlier, and are repeated here.
The word 'taweel' has three meanings:
1) To understand a word in light of one of its connotations, despite the fact that this connotation is not the primary intent of the word.
2) To explain a word or phrase.
3) The actuality of an event.
With these two definitions in mind, there are five main opinions as to the difference between tafseer and taweel, as follows: 
1) They are equivalent in meaning. This was the opinion of at-Tabaree (d. 310A.H.),
as his commentary of the Qur'aan uses these two terms interchangeably.
2) Tafseer is used in explaining a word which carries only one meaning, whereas
taweel is used in choosing one of the connotations of a word that possesses many
3) According to al-Maatureedee (d. 333 A.H.), when the interpretation is based on
certain knowledge, this is called tafseer, whereas when it is based on personal
reasoning (ijtihaad), it is known as taweel.
4) Aboo Taalib at-Tha'labee held the view that tafseer was the explanation of the
literal meaning of the verse, whereas taweel was the actual intent behind the
verse. For example, the tafseer of the verse,
"Verily, your Lord is ever-Watchful" [89:14]
is that Allaah is aware of all that man does, but the ta'weel is that the verse is a warning to man not to lapse into sins or to belittle the commandments of Allaah.
5) Tafseer is meant to give the meanings of the individual words in a verse, whereas taweel gives the meaning of the verse as a whole.
There is no one correct opinion amongst these five, since various authors use these two words in all of these meanings. However, the most common understanding in modern usage of the two words is the second one, namely that tafseer is used to explain the meaning or intent of a verse which has only one connotation, whereas taweel is used when one of the possible connotations of a verse or word is chosen over the others due to external factors.
II. The Necessity and Importance of Tafseer
The question arises: Why is there a need for tafseer? After all, does not Allaah say in the Qur'aan:
"Verily this Qur'aan leads to the path that is most right" [17:9]
and thus everybody can find the Straight Path through this Book? And is not the Qur'aan a complete source of guidance in and of itself, as it says,
"And We have sent down the Book to you as an explanation for everything, a guidance, a mercy and glad tidings for Muslims" [16:89]:
Indeed, it is true that anyone who approaches the Qur'aan with a pure heart, seeking the guidance of Allaah, will find it. As Allaah says,
"This (Qur'aan) is a declaration for mankind, a guidance and an admonition for those who ward off evil" [3:138]
But this in no way implies that a person who is unaware of the numerous hadeeth of the Prophet (SAW) in explaining the Qur'aan, and of the reasons behind the revelation of specific verses, and of the intricacies of Arabic grammar and principles of rhetoric, and of the various qiraaat, and of the knowledge of the abrogated rulings, and of all of the other topics of 'uloom al-Qur aan will benefit from the Qur'aan to the same degree that a person who does know these facts will. For example, an Arabic linguist or grammarian might be able to see a certain wisdom behind the phrasing of a verse that the average person may not. A person specialised in the topics of 'uloom al-Qur aan will be better able to grasp the intended meanings of a verse, and derive rulings from it, in contrast to the average layman, who is not qualified to derive rulings from the Qur'aan.
As-Suyootee also discusses the necessity of 'tafseer in his al-Itqaan. He begins by stating that it is a known fact that Allaah communicates with man in a way that the will be able to understand. This is the reason that every messenger has been sent in the language of his people. However, there are three basic reasons why tafseer is necessary despite these facts. First of all, Allaah uses the most clear, eloquent and concise language, and in doing so the meaning is clear to those who are well-grounded in the Arabic language, but not so clear to those who are not. Secondly, the Qur'aan itself does not always mention the events or references for which each particular verse was revealed, and these must be known in order for the verse to be fully and totally understood. Lastly, some words may have multiple meanings, and it is the job of the person that does tafseer to explain what is meant by the word.
It can be said that the purpose of tafseer is to elaborate the principles which the Qur'aan came to clarify:
1) The elaboration of a perfect set of beliefs with regards to the Creator, and the relationship of the created with the Creator.
2) The perfection of personal conduct and good morals.
3) The establishment of a set of laws and code of conduct to govern individual and
4) The establishment of laws governing societal and political dealings between com
munities and nations.
5) The narrations of the history of the previous nations, so that the pious among
them may be followed, and the impious to act as a warning.
6) To give the good news of Paradise and the blessings in store for the believers, and
the evil tidings of the punishment of Hell in store for the disbelievers.
7) To prove the truthfulness of the Prophet (SAW), and this is done by explaining the
miraculous nature of the Qur'aan (i'jaaz).
Therefore, the job of a mufassir is to explain all of the above points to mankind.
From the above discussion, the importance of tafseer should become apparent. The science of tafseer is meant to explain to mankind the Book that has been revealed to them from Allaah. The Qur'aan is like a treasure trapped in a glass receptacle; mankind can view and benefit from this treasure, but they are in need of tafseer, for tafseer acts like the key that unlocks the treasure, so that mankind can benefit from it to the greatest possible extent. Iyaas ibn Mu'aawiyah (d. 122 A.H.) said, "The example of a people who recite the Qur'aan and do not know its explanation is like a group of people who have a written message from their king that comes to them during the night, and they do not have a lamp. Therefore, they do not know what is in the message. The example of one who knows tafseer is like a person who comes to them with a lamp and reads to them what is in the message." 6 And the Successor Sa'eed ibn Jubayr (d. 95 A.H.) said, "Whoever recites the Qur'aan and does not explain it, is like an ignorant person.
(The science of tafseer) is the most honourable of all sciences for three reasons. The first reason is with respect to its topic. It deals with the Speech of Allaah, which contains every kind of wisdom and virtue. It contains pronouncements about what has passed, reports of what will happen and judgements concerning what happens between the people. Its wonders never cease. The second reason is with respect to its goal. Its goal is to lead mankind to the firm handhold of Allaah, and to the true happiness, one that does not end. The third reason is with respect to the great need for this science. Every aspect of this religion and this world, in the near or distant future, is in need of the sciences of the sharee'ah and knowledge of the religion, and this knowledge can only be obtained through the understanding of the Book of Allaah. 
Apart from these reasons, the Qur'aan itself commands its readers to ponder over it, and to reflect upon its meanings, for it says,
"(This is) a Book which We have sent down to you, full of blessings, so that they may ponder over its verses, and that men of understanding may remember) [38:29]
It is the science of tafseer which is the fruit of 'pondering over its verses.'