Federal statutes are laws enacted by Congress. When Congress passes a law, the law is given a public law number. For example, P.L. 101-194 denotes the 194th law passed by the 101st Congress.
Public laws are first printed as slip laws (that is, individual copies of each law) and then published in chronological order (that is, by public law number) in the United States Statutes at Large. Eventually all session laws are codified by subject matter in the United States Code.