What is the difference between common law and civil law?

The primary difference between common law and civil law is that in common law jurisdictions, published court decisions are of primary importance to understanding legal rules. In civil law jurisdictions, published rules or statutes are of primary importance. Common law's origins are found in England, where court decisions filled in gaps in "writs" issued by the monarch. Civil law, by comparison, traces its origins to codes of laws compiled beginning around the time of the Roman Empire. 

While there are around 150 countries that largely follow a common law model, and 80 that follow a civil law model, the reality is that many jurisdictions incorporate elements of both common law and civil law. Even within a country, different models may apply.