Procedural law comprises the rules by which a court hears and determines what happens in civil or criminal proceedings. The rules are designed to ensure a fair and consistent application of due process (in the U.S.) or fundamental justice (in other common law countries) to all cases that come before a court.
Lawyers distinguish procedural law from "substantive law." Substantive law refers to the actual claims and defenses whose validity is tested through the procedures of procedural law.
See: Legal technicality, civil procedure, criminal procedure