Law refers to a system of rules that regulate the conduct of a community, and is often enforced by a controlling authority through penalties.
When people refer to the "law" of the United States, they most likely mean its law system. The U.S. employs a common law system, which means the laws are typically derived from judicial decisions instead of statutes. A common law system relies on decisions made by judges on cases that have come to trial, and the compilation of these decisions is called case law. Other countries, such as Japan, employ a civil law system, which relies on codes that explicitly specify the rules that judges must follow in order to come to a decision. In such a system, decisions are often not subject to individual interpretation.
Yet another distinction of law is that between procedural law and substantive law. "Procedural law" refers to the guarantees of specific procedural methods and rules, and is distinguished from "substantive law," which refers to the rights and duties of everyday conduct, such as those related to contract law and tort law.
Additionally, international law is "composed only of those rules that States universally abide by, or accede to, out of a sense of legal obligation and mutual concern." (See Flores v. Southern Peru Copper Corporation)