Where civil-law countries build their law from comprehensive codes, common-law countries build it from what judges have decided. Its engine is stare decisis — precedent: a court's ruling binds later courts facing similar facts, so the law accumulates case by case. Born in England after 1066 and carried by the British Empire to the United States, Canada, Australia, India, and much of Africa and the Caribbean — often displacing indigenous legal traditions — common law today governs roughly a third of humanity. Its promise is consistency; its recurring worry is the power it grants to unelected judges.
In much of the world, when you want to know what the law is, you look it up in codes that a legislature wrote. In the common-law world, you read what judges decided. There is no single code that holds the whole law of negligence or contract. Instead the law lives inside thousands of past rulings, each one binding on the courts that come after it, so the law is built case by case, like a coral reef rising from countless small deposits. It began with royal judges riding the roads of medieval England, and the British Empire later carried it to roughly a third of humanity. Its central promise is consistency: like cases should be decided alike. Its central worry is the power it hands to judges who answer to no electorate.