Federal laws are first published in chronological order in United States Statutes at Large (Stat.). Most of them (those with permanent and continuing effect) are codified in the United States Code (U.S.C.).
Even though most of us use the U.S.C. for research, you often see and hear citations to the original public laws—section 8(a)(a) of the National Labor Relations Act, section 703 of the Civil Rights Act, and so on. Why?
Congress has enacted some titles of the United States Code into positive law; other titles are just "prima facie evidence" of the law. For many laws, the original session law is the official version—and those laws are referred to by the section numbers in the original law. If there is any variance between the U.S.C. and Stat. versions of a law, the Stat. version controls. As a practical matter, you will often use and cite the U.S.C. version, but you need to be aware of the Stat. version.
Some statutes that are cited by their original section numbers have been amended a lot. For instance, the Social Security Act, originally enacted in 1935, has been amended hundreds of times. Here's just the beginning of a list (from the U.S.C.A. Popular Names Table on Westlaw):
It would be a big nuisance to have to sort through all those enactments in Stat. every time you wanted to refer to a given section. That's why the House of Representatives Office of the Legislative Counsel has for many years created compilations of major acts (including the Social Security Act) that merge all the amendments. These compilations are just a convenience—the official source is still Stat.—but who doesn't value a little convenience now and then?
Recently the Office of the Legislative Counsel has posted PDFs of many of the most requested compilations. If you're working in an area of law, you can download the statutes you refer to most often.
The Patient Protection and Affordable Care Act, which was just enacted in 2010, has already been amended five times, so the compilation (which includes the amendments) can save you a lot of time.
The compilations include statutes from many fields, including:
- securities (Securities Act of 1933, Securities Exchange Act of 1934, Sarbanes Oxley, Dodd-Frank)
- employment and labor (Civil Rights Act of 1964, National Labor Relations Act, Americans with Disabilities Act)
- environmental law (Superfund Act (CERCLA), Clearn Air Act, Clean Water Act)
- consumer law (Consumer Credit Protection Act, Consumer Product Safety Act, Fair Credit Reporting Act)
If you want to read more about codification of federal statutes, see Mary Whisner, The United States Code, Prima Facie Evidence, and Positive Law, 101 Law Libr. J. 545 (2009).