Definition of Ijtihâd
Linguistically ijtihâd means: to expend efforts in order to reach some difficult matter. Technically it means: expending efforts to arrive at a Sharî'ah ruling. And the Mujtahid is the one who expends efforts for this purpose.

Conditions for Ijtihâd
Being a mujtahid has conditions, from them:-

[1] That he knows the Sharî'ah proofs which he needs in his ijtihâd - such as the âyât (verses) and ahâdîth pertaining to rulings.

[2] That he knows what relates to the authenticity or weakness of a hadîth, such as having knowledge of the isnâd (chain of narration) and it's narrators, and other than this.

[3] That he knows an-nâsikh (the abrogating) and al-mansûkh (the abrogated), and the places where there is ijmâ' (consensus) - such that he does not give a ruling according to something that has been abrogated, nor give a ruling that opposes the (authentically related) ijmâ'.

[4] That he knows from the proofs that which causes the rulings to vary, such as takhsîs (particularisation), or taqyîd (restriction), or it's like. So he does not give a judgement which is contrary to this.

[5] That he knows the Arabic language and usûlul-fiqh (fundamentals and principles of jurisprudence), and what relates to the meanings and indications of particular wordings - such as al-'âm (the general), al-khâss (the particular), al-mutlaq (the absolute and unrestricted), al-muqayyid (the restricted), al-mujmal (the unclarified), and al-mubayyin (the clarified), and it's like - in order that he gives rulings in accordance with what this demands.

[6] That he has the ability to extract rulings from the evidences.

And ijtihâd may be split up, such that it may be undertaken in one particular branch of knowledge, or in one particular issue.

What is Essential for the Mujtahid
It is essential that the Mujtahid strives in expending his efforts to arrive at knowledge of the truth, and to give rulings in accordance to what is apparent to him. If he is correct, then he has two rewards: one for his ijtihâd, and the other for arriving at the truth - since arriving at the truth means that it is manifested and acted upon. If, however, he is mistaken, then he has a single reward, and his error is forgiven for him - as he sallallâhu 'alayhi wa sallam said: “When a judge judges and strives and is correct, then he has two rewards. If he judges and strives and errs, then he has a single reward.” [2] If the ruling is not clear to him, then he must withhold - and in such a case, taqlîd is permissible for him, due to necessity.

Taqlîd - its Definition
Linguistically, taqlîd means: Placing something around the neck, which encircles the neck. Technically it means: Following he whose sayings is not a hujjah (proof).

Excluded from our saying, “Following he whose saying is not a proof.” is: following the Prophet sallallâhu 'alayhi wa sallam, following the ijmâ' and also following the saying of the Sahâbî - for those who consider the saying of a single Sahâbî to be a proof. So following any of these is not called taqlîd, since there is a proof for doing so. However this type of following is sometimes referred to as taqlîd in a very metaphorical and loose sense.

The Place of Taqlîd
Taqlîd is done in two cases:

Firstly: when the muqallid is an 'âmî (a common person) who does not have the ability to acquire knowledge of the Sharî'ah ruling by himself. So taqlîd is obligatory upon him, due to the saying of Allâh - The Most High, “Ask the people of knowledge if you do not know.” [3] So he does taqlîd of one whom he considers to be a person of knowledge and piety. If there are two such people who are equal in his view, then he chooses any one of them.

Secondly: The mujtahid when he encounters a new situation, for which an immediate solution is required, but it is not possible for him to research into this matter. So in this case he is permitted to perform taqlîd.

Some stipulate as a condition for the permissibility of taqlîd, that the matter is not from the fundamentals of the Dîn - those matters which must be held as 'aqîdah - since matters of 'aqîdah require certainty, whereas taqlîd only amounts to dhann (knowledge which is not certain).

However the correct saying in this matter is that this is not a condition, due to the generality of his - the Most High's - saying, “Ask the people of knowledge if you do not know.” And this verse is in the context of affirming the Messengership - which is from the fundamentals of the Dîn. And also because the common person cannot acquire knowledge of the Sharî'ah rulings with it's proofs by himself. So if he is unable to arrive at the truth by himself, then nothing remains for him except taqlîd, due to the saying of Allâh - the Most High, “Fear Allâh as much as you can.” [4]