Brief Answer
A fatwa is a universally applicable religious law concerning a particular issue that is derived from the four sources of Islamic law (the Qur`an, the sunnah, ijma’ (consensus), and ‘aql (the intellect)) using the methodology of ijtihad (juristic interpretation).
A hukm on the other hand, is a law issued by a qualified mujtahid (jurist) who is the head of an Islamic government. Such a leader, by keeping in mind the general principles of divine law, the Islamic systems[396], and prevailing conditions, issues a hukm (legal ruling) for all people, a specific contingency, or an individual.
From the divine perspective, obedience of both a fatwa and a hukm is obligatory. The only difference in this regard is that a fatwa is only legally binding on the mujtahid who issues it and his muqallid (one who imitates his rulings), while the hukm is binding on all groups or individuals enumerated in the hukm.