The Congress shall have power to . . . promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries
The Copyright and Patent Clause of the U.S. Constitution (Article I, Section 8, Clause 8)

Copyright Basics (Circular 1) is recommended reading for understanding copyright.

U.S. Copyright Office Home PageA first stop for anyone interested in copyright. Learn about copyright from information circulars, keep current with new legislation, download registration forms, or search the Office's online copyright records.Information CircularsA useful resource; many publications have been revised to reflect changes made by recent legislation. Of special interest are Circular 1 - Copyright Basics, 15a - Duration of Copyright: Provisions of the Law Dealing with the Length of Copyright Protection, 21 - Reproductions of Copyrighted Works by Educators and Librarians, Fair Use (FL102), and New Terms for Copyright Protection (SL15).New/Pending Copyright LegislationLink to the text of current legislation here.NewsNetThe Copyright Office's e-mail service "for people involved in the field of copyright." NewsNet "alerts subscribers to hearings, deadlines for comments, new and proposed regulations, new publications, and other copyright-related subjects of interest." Current and back issues are also available here.U. S. Patent and Trademark OfficeUSPTO administers the laws relating to patents and trademarks and advises the Secretary of Commerce, President, and administration on patent, trademark, and copyright protection.
Although it's beyond the scope of this guide to cover patents and trademarks, those interested in exploring this aspect of intellectual property* should see Patents>General Information Concerning Patents and Trademarks>Basic Facts About Trademarks. (Many of the sites in the Copyright Resources: Legal section of this guide also offer patent and trademark information.)
*"A category of intangible rights protecting commercially valuable products of the human intellect. The category comprises trademark, copyright, and patent rights, but also includes trade-secret rights, publicity rights, moral rights, and rights against unfair competition." Black's Law Dictionary, 7th Edition.