In general, legal systems can be split between civil law and common law systems. The term civil law should not be confused with civil law as a group of legal subjects, as distinct from criminal or public law. A third type of legal system—still accepted by some countries—is religious law, based on scriptures and interpretations thereof. The specific system that a country is ruled by is often determined by its history, connections with other countries, or its adherence to international standards. The sources that jurisdictions adopt as authoritatively binding are the defining features of any legal system. Yet classification is a matter of form rather than substance, since similar rules often prevail.

Law is a system of rules, enforced through a set of institutions,used as an instrument to underpin civil obedience, politics, economics and society. Law serves as the foremost social mediator in relations between people. Writing in 350 BC, the Greek philosopher Aristotle declared, "The rule of law is better than the rule of any individual."