Definition of Knowledge
- The opposite of ignorance, which is: To understand something with definitive understanding.
- Some of the people of knowledge have said: It is to know (something) and it is the opposite of ignorance, while others from amongst them have said: Certainly, knowledge is more clear than to (simply) know.
What we mean here is knowledge of the Sharee'ah, and what is meant by this is: Knowledge which Allaah has revealed to His Messenger from the evidences and guidance.
Therefore, knowledge which has praise and commendation attached to it is knowledge of the revelation, that being, only knowledge which Allaah has revealed.
The Prophet (salAllaahu 'alayhi wa sallam) said:
"Whoever Allaah wishes good for, He grants him understanding of the religion"
The Prophet (salAllaahu 'alayhi wa sallam) also said:
"Certainly, the Prophets have not bequeathed deenar nor dirham, rather they have bequeathed knowledge, so whoever takes it has taken abundant good fortune."
From that which is known, is that what the Prophets have bequeathed is the knowledge of the Sharee'ah of Allaah and nothing other than it. So, the Prophets have not bequeathed for man knowledge of manufacturing and that which is related to it; rather when the Messenger (salAllaahu 'alayhi wa sallam) arrived in Madeenah, he found the people grafting the date palm trees. Having seen how tired they had become, he said to them something to the meaning of: there is no need for this, meaning grafting. Upon hearing this, they left the process of grafting, and as a result the date palm trees deteriorated in their yield. The Prophet (salAllaahu 'alayhi wa sallam) then said to them:
"You have better knowledge (of a technical skill) in the affairs of your world."
Were it that such knowledge carried with it an element of praise then the Messenger (salAllaahu 'alayhi wa sallam) would have been more knowledgable than the people about it, this is because the one who is most praised and blessed with knowledge and action is the Prophet (salAllaahu 'alayhi wa sallam).
Therefore, knowledge of the Sharee'ah is that which has an element of praise attached to it and commendation for one who acts upon it.
However, having said this, I do not deny that which other knowledge there is benefit, rather, it must possess a double-edged benefit: If it assists in obedience to Allaah and leads to the success and victory of the Religion of Allaah, and the servants of Allaah are able to benefit from it, then that is good and beneficial, and learning it may well be obligatory at some time should it fall under the statement of Allaah (Subhanahu wa Ta'ala):
And make ready against them all you can of power, including steed of war... 
It has been mentioned by many from amongst the people of knowledge that seeking knowledge of manufacturing is fard kifaayah, and that is for example, because it is imperative the people have (utensils and) containers to cook with and drink from, and similar issues with which they can seek benefit. So if there were none who came forward to establish such a process, then the learning of it would become fard kifaayah and this is a controversial issue amongst the people of knowledge. However, I wish to say that knowledge which has an element of praise and commendation attached to it is knowledge of the Sharee'ah, which is understanding the Book of Allah and the Sunnah of His Messenger (salAllaahu 'alayhi wa sallam), and that which is other than this is either a means to something good or a means to something evil. Therefore its ruling is based upon what that particular knowledge is a means to, either good or evil.
Taken from; 'The Book of Knowledge', by shaykh 'Uthaymeen.
 Saheeh al-Bukhaaree - The Book of knowledge; Saheeh Muslim - The Book of Zakaah
 Abu Daawood - The Book of Knowledge; at-Tirmidhee - The Book of Knowledge
 Saheeh Muslim - The Book of Virtues
 The Noble Qur'aan - Soorah al-Anfaal, Ayah 60
 A collective obligation, whereby if some of the community uphold this obligation, then this suffices on behalf of the rest. (Translator)