Legal definitions (from the U.S. Copyright Office except as noted)
U.S. Code Title 17 (an excerpt from Cornell University)
Copyright Basics -- an e-pamphlet from the U.S. Copyright Office explaining basic definitions.
what copyright is -- an excerpt from Copyright Basics
what works are protected -- an excerpt from Copyright Basics
what is not protected (see also exemptions such as sections 105 and 120) -- an excerpt from Copyright Basics
Historical origins -- by Dale C. Mead. An informative presentation from an interesting perspective. Dale Mead is an attorney and librarian currently employed as an Information Architect at Nortel Networks. See here for more about his work.
Look also at the early history of the US statute.
To a broader context