The following excerpt is taken from FDsys.
Public and Private Laws
Public and private laws are prepared and published by the Office of the Federal Register (OFR), National Archives and Records Administration (NARA).
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.
Prior to publication as a slip law, OFR also prepares marginal notes and citations for each law, and a legislative history for public laws only. Until the slip law is published, through the U.S. Government Printing Office (GPO), the text of the law can be found by accessing the enrolled version of the bill.
Note: 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 (1 U.S.C. 113).
What is the difference between a public and private law?
Public and private laws contain the following information in either the header or side notes:
At the end of each session of Congress, the slip laws are compiled into bound volumes called the Statutes at Large, and they are known as "session laws." The Statutes at Large present a chronological arrangement of the laws in the exact order that they have been enacted.
Every six years, public laws are incorporated into the United States Code, which is a codification of all general and permanent laws of the United States. A supplement to the United States Code is published during each interim year until the next comprehensive volume is published. The U.S. Code is arranged by subject matter, and it shows the present status of laws with amendments already incorporated in the text that have been amended on one or more occasions.