Case law is also known as decisional law. It is that body of reported judicial opinions in countries that have common law legal systems that are published and thus become precedent, i.e. the basis for future decisions.
This law is judge-made law that interprets prior case law, statutes and other legal authority -- including legal scholars such as the Corpus Juris Secundum, Halsbury's Laws of England or the doctrinal writings found in the Recueil Dalloz and law commissions such as the American Law Institute.
Case law officially plays a minor role compared to the status of the civil code. Nevertheless, judicial interpretation of the civil code, interpreting the legal meaning of the code's provisions, clarifying them, and providing for unexpected developments, is often referred to as a jurisprudence constante.
 However, via precedents, case law regulates how laws are to be understood, based on how prior cases have been decided. It governs the influence court decisions have on future cases. Common law systems, in spite of most civil law systems, follow the doctrine of stare decisis in which lower courts usually make decisions consistent with previous decisions of higher courts.
Case law plays the different role in civil and common law traditions. It creates differences in the way that courts render their decisions. The underlying principle is explained in details by common law courts with many citations to previous decisions and other authority. But decisions in the courts of most civil law jurisdictions are generally very short, referring only to the statutes used. In these civil law jurisdictions, the tradition is that the reader should be able to deduce the logic from the decision. And this is the reason for this difference. Courts in civil law jurisdictions also render their decisions so that, in some cases, it is somewhat difficult to apply previous decisions to the facts presented in future cases.
In common law traditions law professors play a much smaller role in developing case law than professors in civil law traditions. And this is another difference. Unlike judges, academics do much of the exposition of the law in civil law traditions. The reason for this is that court decisions in civil law traditions are brief and not amenable to establishing precedent. This is called doctrine and may be published in treatises or in journals such as Recueil Dalloz in France.
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