Sharia is the law that regulates the daily life of a Muslim and serves as a guide for living by Islamic principles. As Islam is not only a belief system but a complete way of life, the law covers all aspects of living including moral, spiritual, intellectual, physical, economical, political, etc. Sharia law is derived by scholars through interpretations of Islam's canonical texts, the Qur'an and Hadith (sayings and actions of Muhammad). As with any legal system the interpretations can range across the liberal-conservative spectrum, and opinions on the meanings and applications will often differ. Therefore sharia law is essentially an effort to comprehend God's instructions and apply them in daily life.
Since the interpretations are made by humans, they are subject to error and even perversion. This occurs when unqualified, ignorant and/or corrupted individuals make the interpretation. For instance the KKK perverts Christian texts to conjure up false rulings. The same can be said for certain Muslim groups and so-called "Islamic states" regarding Islamic texts.
While there is no Pope in Islam to serve as the authoritative interpreter, there are recognized institutions and scholars whom analyze, discuss, deliberate and arrive at reasonable rulings through a process known as "fiqh" (Islamic jurisprudence). This nuanced and sophisticated science operates on the premise that the Islamic texts are fluid and dynamic; that anything outside of the very basic tenets can, and should, be interpreted according to the particular time, place and culture. So if any version of sharia advocates brutality, injustice, extremism, terrorism, etc., the question must be asked: are such rulings endorsed by mainstream Islamic scholars, or are they being posited by those devoid of a true understanding of the religion, or the wisdom to apply it?