Sharīʿah, also spelled Sharia, the fundamental religious concept of Islam, namely its law, systematized during the 2nd and 3rd centuries of the Muslim era (8th–9th centuries CE).
Total and unqualified submission to the will of Allah (God) is the fundamental tenet of Islam: Islamic law is therefore the expression of Allah’s command for Muslim society and, in application, constitutes a system of duties that are incumbent upon a Muslim by virtue of his religious belief. Known as the Sharīʿah (literally, “the path leading to the watering place”), the law constitutes a divinely ordained path of conduct that guides Muslims toward a practical expression of religious conviction in this world and the goal of divine favour in the world to come.
In classical form the Sharīʿah differs from Western systems of law in two principal respects. In the first place the scope of the Sharīʿah is much wider, since it regulates an individual’s relationship not only with one’s neighbours and with the state, which is the limit of most other legal systems, but also with God and with one’s own conscience. Ritual practices, such as the daily prayers, almsgiving, fasting, and pilgrimage, are an integral part of Sharīʿah law and usually occupy the first chapters in the legal manuals. The Sharīʿah is also concerned as much with ethical standards as with legal rules, indicating not only what an individual is entitled or bound to do in law but also what one ought, in conscience, to do or refrain from doing. Accordingly, certain acts are classified as praiseworthy (mandūb), which means that their performance brings divine favour and their omission divine disfavour, and others as blameworthy (makrūh), which means that omission brings divine favour and commission divine disfavour; but in neither case is there any legal sanction of punishment or reward, nullity or validity. The Sharīʿah is not merely a system of law, but a comprehensive code of behaviour that embraces both private and public activities
Offenses against another person, from homicide to assault, are punishable by retaliation (qiṣāṣ), the offender being subject to precisely the same treatment as his victim. But this type of offense is regarded as a civil injury rather than a crime in the technical sense, since it is not the state but only the victim or his family who have the right to prosecute and to opt for compensation or blood money (diyah) in place of retaliation.
For six specific crimes the punishment is fixed (ḥadd): death for apostasy and for highway robbery; amputation of the hand for theft; death by stoning for extramarital sex relations (zinā) where the offender is a married person and 100 lashes for unmarried offenders; 80 lashes for an unproved accusation of unchastity (qadhf) and for the drinking of any intoxicant.
See also: http://www.shariawatch.org.uk/?q=content/can-islamic-law-evolve