Sharia is often referred to as Islamic law, but this is wrong, as only a small part is irrefutably based upon the core Islamic text, the Koran. Correct designations would be "Muslim Law", "Islam-inspired", "Islam-derived," or even "the law system of Muslims."
17 ...then we gave you a Sharia in religion, follow it, and do not follow the lust of those who do not know...
But despite this, many parts of the Sharia have no or little importance in most Muslim societies, except in those that have gone through a phase of Islamization (Sudan, Iran, Saudi Arabia, and to some extent Libya). But the Sharia has much importance in domestic judicial fields like family, marriage and inheritance.
In Sunni Islam, there are four schools, madhhab, which all coexist in peace. No war has ever been fought over the issue of different schools, and students of religious subjects in most Muslim countries have to learn about all four schools. It is in many cases permissible to use a law from another school, if one feels that it is more appropriate. All schools have a lot in common, but there are many cases where the same act is regarded very differently. For the very same issue the schools can stretch from classifying things to be everything between forbidden and meritorious.