Public and private laws are also known as slip laws. A slip law is an official publication of the law and is competent evidence admissible in all state and Federal courts and tribunals of the United States. Public laws affect society as a whole, while private laws affect an individual, family, or small group.
After the President signs a bill into law, it is delivered to the Office of the Federal Register (OFR), National Archives and Records Administration (NARA) where it is assigned a law number, legal statutory citation (public laws only), and prepared for publication as a slip law. Private laws receive their legal statutory citations when they are published in the United States Statutes at Large.
http://www.gpo.gov/fdsys/browse/collection.action?collectionCode=PLAW