Case law (precedential law) is the body of judge-made law and legal decisions that interprets prior case law, statutes and other legal authority -- including doctrinal writings by legal scholars such as the Corpus Juris Secundum, Halsbury's Laws of England or the doctinal writings found in the Recueil Dalloz and law commissions such as the American Law Institute.
In the civil law tradition, case law formally plays a minor role compared to the status of the civil code; however, judicial interpretation of the civil code, interpreting the legal meaning of the code provisions, clarifying them, and providing for unforeseen developments, is often referred to as a juriprudence constante. In France, the jurisprudence constante of the Cour de cassation (for civil and penal cases) or the Conseil d'État (for administrative cases) is in practice equivalent to case law, and is of considerable import in certain domains such as labor law or administrative law. In particular, the Conseil d'État and the Constitutional Council have distinguished "fundamental principles" that statutes and regulations must follow, even when those principles were not explicitely written in statutes.
In the common law tradition case law regulates, via precedents, how laws are to be understood, based on how prior cases have been decided. Case law governs the impact court decisions have on future cases. Unlike most civil law systems, common law systems follow the doctrine of stare decesis in which a lower court usually make decisions which are consistent with previous decisions of higher courts. However, if a lower court judge acts against precedent and the case is not taken to appeal it will still stand. Generally speaking there is no direct oversight that appeal courts have over a court of record. This may occur more frequently than has been documented as an appeal is usually quite expensive to prepare. Some judges are also known to rule against precedent on principle. A judge (or even an interim appeal court) may rule against a precedent that is outdated; she may feel the decision needs to be overturned due to more sophisticated legal reasoning. Such a judge may wish to help the law evolve by ruling against precedent and forcing the case to appeal. Then, the appeal court will have an opportunity to review the lower court's decision and may adopt the lower court's reasoning thereby overturning previous cases. This may also happen several times as the case works its way through intermediate appellate jurisdictions. A famous example of this evolution in jurisprudence was by Lord Denning, first of the High Court of England and Wales, later of the Court of Appeal in his development of the concept of estoppel starting in the world renowned High Trees case: Central London Property Trust Ltd. v. High Trees House Ltd.  K.B. 130.
The different roles of case law in civil and common law traditions creates differences in the way that courts render their decisions. Common law courts generally explain in detail the rationale behind their decisions with numerous citations to previous decisions and other authority (called ratio decidendi). By constrast, decisions in the courts of most civil law jurisdictions are generally very short, referring only to the statutes used. The reason for this difference is that in these civil law jurisdictions the traditions is that the reader should be able to deduce the logic from the decision. They also render their decisions so that in some cases it is somewhat difficult to apply previous decisions to future ones. Some so-called civil law jurisdictions such as Scotland, Quebec and Louisiana do not follow these traditions, as they are jurisdictions that have been influenced heavily by the Anglo-American common law tradition, however their substantive law is firmly rooted in the civil law tradition. Because of their position between the two main systems of law these types of legal systems are sometimes referred to as mixed systems of law.
Another difference is that law professors in common law traditions play a much smaller role in developing case law than professors in civil law traditions. Because court decisions in civil law traditions are brief and not amenable to establishing precedent, much of the exposition of the law in civil law traditions is done by academics rather than by judges, this is called doctrine and may be published in treatises or in journals such as Recueil Dalloz in France. In the common law the practice has evolved in the other direction, thus at the turn of the twentieth century it was very rare to see an academic writer quoted in a legal decision (except perhaps for Blackstone). Today academic writers can be cited in legal decisions as authority, often they are cited when judges are attempting to find reasoning that has yet to be adopted by any other court or because their restatement of the law is more compelling than the ratio of precedent. Thus common law systems are adopting one of the approaches long common in civil law jurisdictions.
In federal or multi-juristictional law systems there may exist conflicts between the various lower appellate courts. Sometimes these differences may not be resolved and it may be necessary to distinguish on how the law has been applied in one district, province, division or appellate department. Usually only an appeal accepted by the court of last resort will resolve such differences and for many reasons such appeals are often not undertaken.