I have seen references to the principles of fiqh by
Imam Malik as:
An explicit text (nass) of the Qur'an.
A apparent (dhahir) text which is (an apparent meaning derived from a text which is general and non-specific).
An indicative (dalil) text, which may have an interpretation which diverges from its obvious meaning.
An implicit (mafhum) text, which has an added meaning coinciding with its obvious meaning.
An expositive (tanbih) text, which reports the underlying reason for a judgement (like the statement 'it is filth').
The same five categories in respect of the Sunnah;
Consensus (ijma').
Analogy (qiyas).
The practice of the people of Medina ('amal ahli'l-madina).
A statement of a Companion (qawl as-sahabi)
Judicial preference (istihsan)
Blocking of the means (sadd adh-dhara'i').
taken from http://ourworld.compuserve.com/homepages/Abewley/usul.htmlwith similar writeups elsewhere.Also, I have always been told that, traditionally within Islamic jurisprudence, all things are permissable unless a prohibition against them can be found using the principles of fiqh. The extremists frequently turn this on it's head, and forbid everything not expressly allowed in the Qur'an and Sunnah. These people also teach believers to interpret the Qur'an and Sunnah on thier own, without regard to scholarly traditions. This is dangerous, as the order and circumstances of sayings and verses provide the clues as to their full meaning. Ijtihad is not a matter for the reckless, nor for the unwise. This is one of the reasons that it is important to know the isnad of a shaykh when requesting a fatwa.