Legal Definitions: A Research Guide for Congressional Staff

"This report is designed to introduce congressional staff to examples of legal and nonlegal sources, including statutes, legislation, and dictionaries, for researching legal definitions. It includes governmental sources, such as the United States Code (U.S. Code), the Code of Federal Regulations (C.F.R.), and congressional legislation. Proprietary information sources are also included..."
Legal Definitions 

From Slip Law to United States Code: A Guide to Federal Statutes for Congressional Staff

"This report provides an overview of federal statutes in their various forms, as well as basic guidance for congressional staff interested in researching statutes. When a bill becomes a law, the newly enacted statute may amend or repeal earlier statutes or it may create a new or “freestanding” law. Either way, these new statutes are first printed individually as “slip laws” and numbered by order of passage as either public laws, or less frequently, private laws. Slip laws are later aggregated and published chronologically in volumes known as the United States Statutes at Large (Statutes at Large). Statutes of a general and permanent nature are then incorporated into the United States Code (U.S. Code), which arranges the statutes by subject matter into 54 titles and five appendices.

Statutes may be updated and published as amended public laws. As the statutes that underlie the U.S. Code are revised, superseded, or repealed, the provisions of the U.S. Code are also updated to reflect these changes. In these instances, the authoritative language remains the enacting statute, or the “base law.” However, some titles of the U.S. Code have been passed into “positive law,” meaning the law exists as it does in the U.S. Code and the title itself is the authoritative language. In these instances, it is the U.S. Code sections that are revised, superseded, or repealed, as the underlying statutes have all been revoked..."

Federal statutes

Historical Versions of the United States Code Now Online

"More than 60 years of U.S. laws are now published online and accessible for free for the first time after being acquired by the Library of Congress. The Library has made available the main editions and supplements of the United States Code from 1925 through the 1988 edition

The U.S. Code is a compilation of federal laws arranged by subject by the Office of the Law Revision Counsel of the House of Representatives. The Library’s U.S. Code Collection is fully searchable. Filters allow users to narrow their searches by date, title and/or subject. PDF versions of each chapter can be viewed and downloaded.

The collection is online at loc.gov/collections/united-states-code/. This provides access to editions of the U.S. Code that previously were not available to the public online for free..."
U.S. Code Online