At the end of each session of Congress, the newly-enacted laws (called "slip laws") are compiled into bound volumes called the Statutes at Large, where they are then known as session laws. The Statutes at Large present a chronological arrangement of the laws in the exact order that they have been enacted. In time these statutes at large will be incorporated (codified) into the Federal Code.
Public laws are given a number that identifies which Congress it was enacted in, and the order in which it was enacted.
e.g. National Voter Registration Act of 1993. Pub. L. 103-31. 107 Stat. 77-89.
This represents the 31st law enacted by the 103rd Congress in 1993. It was published in the 107th volume of the Statutes at Large, at pages 77-89.