The Sharia is comparable to what in Judiasm is known as the Talmudic laws. These laws govern religious practices, intercourse, marriage, contracts, interaction with gentiles, what defines a gentile and the list goes on. The laws are comprehensive. Much like Talmudic law, Abrahamic laws or Semitic rational is focused on divine revelation, unlike Greek or Roman laws that found naturalist or humanist or what we can call rationalist grounding. Where the morals and norms of society dictate the laws governing society.
Christianity did not develop this system of laws, in essence the reformation brought Protestant theology to the forefront, Luther in essence confined God to spirituality and constructed a moral system in symbiosis with secular- or rationalist- thought. What we have in the Western world today is this creation, the compartmentalization of law between the private and public sphere. Even Catholicism did not develop laws that encompassed the totality of man's affair. Part of why I believe this is the case is because the life of Jesus did not allow for a that development and more importantly he was a reformer sent by God to reform Judaism, he was Jewish.
Islam for its part had in the Prophet Muhammad a totality of experiences and a set of divine conjunctions governing human affairs. The two in essence- the Quran and the Hadith (collection of the sayings, teachings and actions of the Prophet Muhammad, the Sunnah, way of the Prophet) were collected and formed the basis of what we have today as the Sharia.
However, the Sharia is not in itself "complete" in that the Quran and Sunnah are the sources of the "laws" that make up the Sharia and are "general directives" that do not provide the methodology and procedural guidelines that ensure the appropriate use of the source. How do we know when what Hadith comes into play and not another, how the ruling for stealing another's property is detailed and what its punishment is?
That comes from Fiqh and Usul al-Fiqh. Both terms require and in depth look that cannot be done here. However Fiqh is acquired knowledge, that comes from studying and self application and is different from inherent knowledge- or knowledge like that given to the Prophet or inherent with the angel Gabriel. The following passage comes from "Principles of Islamic Jurisprudence" by Mohammad Hashim Kamali and it shines light on where we get the Sharia we have today, gives appriciation to its development and provides us with a connection to the sacrifices made to develop it and for us to preserve it under the attacks from outside and within.
When the Prophet was alive, the necessary guidance and solutions to probelms were obtained either through divine revelation, or his direct ruling. Similarly, during the period following the demise of the Prophet, the Companions remained in close contact with the teachings of the Prophet, and their decisions were mainly inspired by his precedent. Their proximity to the sources and intimate knowledge of events, provided them with the authority to rule on practical problems without there being a pressing need for methodology. However, with the expasion of the territorial domain of Islam, the Companions were dispersed and direct access to them became increasingly difficult. With this, the possibilty of confusion and error in the understanding of the textual sources became more apparent. Disputation and diversity of juristic thought in different quarters accentuated the need for clear guidelines, and the time was ripe for Al-Shafi to articulate the methodology of usul al-fiqh. Al-Shafi came on the scene when juristic controversy had become prevalent between the jurists of Medina and Iraq...This was also a time when the ulama of hadith had succedded in their efforts to collect and document the hadith. Once the fuqaha were assured of the subject-matter of the Sunnah, they began to elaborate the law, and thus the need for a methodology to regulate ijtihad became increasingly apparent. The consolidation of usul al-fiqh as a Sharia discipline was, in other words, a logical conclusion of the compilation of the vast literature of hadith.
Part of the process of developing methodology came from the expansion and the incorporation of non-Arabs. This in part made Islam distant from direct cultural influence that might corrode the message and teachings of the Prophet, in essence this development helped protect Islam in a shell and allowed the various cultures to adapt around Islam. Amazing!