The evidence are of 4, and are used to derive ruling in the shariah they are:
Definition for each one:
Qur'aan: Speech of Allaah
Sunnah: The statements, actions and tactic approval of the Prophet Muhammad (SAW). Tactic approval is basically anything that has been said or doen by the companions of the Prophets in the presence of the Prophet (SAW) in which he didn't rebuke.
Ijma': Scholary Consensus.
Qiyas: Analogical reasoning.
Ijithad is independent reasoning and one who performs ijtihad is a mujtahid and must have permission to do so or otherwise he will misguide others.