Public Laws are the individual laws (a/k/a "statutes" or "session laws") created and enacted by the U.S. government that affect the general population (as opposed to Private Laws, which affect only specified individuals or entities). Public Laws are published officially in Statutes at Large and subsequently Codified in the United States Code.
Recent P.L.s are posted free on FDsys back to the 104th Congress (1995-96); older P.L.s are available on Thomas back to the 93rd Congress (1973-74). GPO Access lets you search and browse titles. With Thomas you pretty much have to know the Bill or P.L. number.
Public Laws are published in the U.S. Code Congressional and Administrative New Service (USCCAN), back to 1941 and, after a year or so, in Statutes at Large all the way back to 1789 (see "Statutes At Large").
On Lexis, you can get newer P.L.s in the Public Law file (GENFED;PUBLAW) back to the 2nd Session of the 100th Congress (1987), and older P.L.s in the Statutes at Large file (LEGIS;STATLG) back to 1798. You can get P.L.s using the formats: 104 pl 365 or 38 stat 717.
Westlaw has public laws in four database: US-STATLRG covers 1798 to 1972; USCCAN-PL covers 1973 to the present; US-PL-OLD covers 1973 to the last completed Congressional term; US-PL covers the public laws from the current Congress. You can "Find" public laws on Westlaw using the format: PL 104-365.
HeinOnline offers subscribers a complete online edition of Statues at Large.
Additional sources for public laws are discussed in the LLSDC's Legislative Sourcebook; scroll down to the section called "Electronic Sources for U.S. Statutes and the U.S. Code."
To get a recent Public Law number, search bill summaries by keyword/subject/etc. on Thomas. For older laws, you can look up the bill number in the CIS/Index to Publications of the United States Congress back to 1970 (LEGIS;CISINX on Lexis), the Congressional Masterfile I from 1978-1980 (LEGIS;CISHIST on Lexis), the CCH Congressional Index back to 1941 or the United States Code Congressional and Administrative News (USCCAN) also back to 1941.
Annotations: If a Federal law has been codified into the United States Code, you can find annotations for each Code section in the United States Code Annotated and the United States Code Service (see "United States Code"). If a Federal law has not been codified, you can find judicial opinions, law review articles and other sources citing a Federal public law by Shepardizing the Statutes at Law citation in the print edition of Shepard's Federal Statute Citations.
Codification: To see where a section of a recent Public Law has been put into the United States Code (USC), you can try to read the Public Law closely to see if it tells you where the section goes. Alternatively, you can .
(a) look in the paper back Tables volumes at the end of the United States Code Annotated (USCA); or
(b) look up the law in the Concordance of Federal Legislation (William S. Hein & Co.), which is designed expressly for this purpose; or
(c) look up the law in the United States Code Congressional and Administrative News (USCCAN) - USC sites are printed in the margins for laws published since 1979; or
(d) do a keyword search in an electronic edition of the USC; or
(e) look up the section by subject in the index to the United States Code Annotated (USCA) or United States Code Service (USCS).
If you know the "popular name" of the law, you can look it up in a Popular Name Table, as discussed in the "Popular Names" section of the United States Code entry.
For a discussion of Federal legislation and Public Laws, see Fundamentals of Legal Research (West).