Session laws are the laws of a jurisdiction, collected in chronological order by date of passage. The session laws of the United States are called Public Laws, abbreviated Publ. L., or P.L. Public Laws are cited like this: Publ. L. 107-56, where the "107" designates the Congress (the 107th Congress, in this case) and the "56" means the 56th law passed by that Congress.
The Public Laws of the United States are collected in a set of books called the Statutes at Large, abbreviated Stat. The Statutes at Large are organized in volumes chronologically and are cited like this: 115 Stat. 272, where 115 is the volume number and 272 is the page number within the volume. Publ. L. 107-56 is found at 115 Stat. 272 in the Statutes at Large.
Codes are collections of the laws of a jurisdiction, that are currently in force. Codes are organized by subject. The United States Code is the code of the laws of the United States. It is organized in 53 titles (for example, Title 42 is Public Health and Welfare), and abbreviated as U.S.C.
The laws of the United States are codified in the United States Code (U.S.C.). The U.S. Code is printed every 6 years (the 2012 Code is the current one), with an annual supplement each year in between for updating. Sources:
What is an "annotated code"? "Annotated" indicates that the editors of the code have added case finding tools, explanatory material, and research resources to the text of the code, to assist the researcher.
Using the U.S. Code: Finding tools in print and in Westlaw (U.S.C.A.) and Lexis Advance (U.S.C.S.)
1. General Title Index and Descriptive Word Index: find statutes by topic
2. Popular Name Tables: find where Public Laws are codified when the information you have is the popular name of the act, e.g. the Clean Air Act. In the last volume of the Descriptive Word Index in print; in separate files in Westlaw and Lexis Advance.
3. Tables Volume: includes
Bloomberg Law has its own Popular Name Table. Also, the Code is browseable by title and can be searched within titles as well.