Session laws are chronological compilations of the laws passed by a particular legislature within each session. All federal statutes are published in chronological order in the Statutes at Large (e.g., 78 Stat. 241). This indicates that the law being cited begins on page 241 of Volume 78.
Why cite to a session law and not a code? There are several situations, including one in which the act has been repealed and is no longer in the code; the act was never codified (only laws of a general and permanent nature are put into the code); or the act was broken up and placed in different parts of the subject-based code (in this case, if citing to that law as a whole, it would be appropriate to cite to the session law as opposed to scattered code provisions).
Cite according to Bluebook Rule 12.4.
Note: session law citations are generally easy to identify because they contain either the number of the Congress or the year of the state legislative session. The other part of the citation indicates which law it was chronologically. For example, this law was the 352nd law passed by the 88th Congress.