Case law (often spelled caselaw) is one source of law in the United States.
The common law system on which the United States is based provides that judges should apply legal principles from prior cases – – precedent – – to cases that present similar facts. For example, if judges in prior cases held that a person cannot form a contract with someone he knows to be intoxicated, judges should apply that legal principle to circumstances where a person tries to form a contract with someone he knows is suffering from another type of condition that would prevent him from thinking clearly.
In some situations, judges are required to follow the decisions of judges in prior cases. Lower courts must follow precedent from superior courts in the same jurisdiction.
In the federal court system, trial level courts are called United States District Courts. The District Courts must follow precedent established by the intermediate appellate courts in their jurisdiction. For example, New York is in the Second Circuit. As a result, New York federal courts must follow precedent established by the Second Circuit Court of Appeals.
The same principle applies in state court systems. New York trial level courts must follow precedent established by appellate courts in the same jurisdiction.