Whereas private laws govern interactions between individuals (like contracts, torts, or domestic relations), public laws refer to the relationship of the state and the general population (like criminal law, constitutional law, and all procedural law).
Example citation: PL 103-412
The first part of the number represents the number of the Congress that enacted the law (the 103rd Congress).
The second part of the number indicates the chronological sequence of enactment (the 412th law enacted by that Congress).
After the President signs a bill into law, it is delivered to the National Archives and Records Administration (NARA) Office of the Federal Register, where editors assign a public law number.
Find United States Public Laws dating from 1933-present in microfilm, paper, and online (Smith KF 50 .U512x,Microforms Micro-10 S504, GPO website)
Search Congress.gov (under "Laws") for Public Laws. Public Laws on the site date back to the 93rd Congress (1973-1974).