To promote the progress of science and the useful arts (useless arts are specifically excluded) by securing for a limited time to authors and inventors the exclusive right to their respective writings and discoveries.
It's clear that the framing fathers sought to maximize the public good by granting a copyright for the sole purpose of ensuring some fair income as either incentive or sustenance to the artist. But it is also clear that the intent was not to grant permanent and indefinite ownership of the intangible concept of Intellectual Property, Indeed as the United States' first patent examiner, Thomas Jefferson wrote:
"He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me."
As copyright law has become increasingly slanted toward the author (or, more precisely to toward corporations like Disney but more on that later) it has grown increasingly out of sync with the original intent, leading Bert Boyce to write:
"Current copyright law... on it's face, in contradiction with the clearly expressed purposes of the authors of the basic law of the United States... A law that meets constitutional requirements should be able to demonstrate that the granting of exclusive rights in some way benefits the authors and does not impede the progress of science..."
Nick Negroponte is more succinct: "copyright law is totally out of date."
Copyright is automatically granted to the author the moment his or her work is fixed in a tangible medium of expression, a definition that is being expanded to include electronic publication explicitly, but at the moment is hopelessly vague. Your web page probably is copywritten, your erotic IRC stanzas probably aren't. The court's reasoning is that your web page is intended to be fixed for a time and your chat messages are generally considered ephemeral.
Copyright grants the author exclusive rights to reproduction or exhibition of the protected work, excepting certain ill defined exceptions which fall under the term "fair use."
Copyright was originally granted for 14 years with a 14 year extension. The 1976 revision of article 17 extended the law to 50 years after the death of the creator, 75 years for corporations.
Sonny Bono, the ex-congressman who has risen from the grave to sponsor HR 2281 along with his wife, is the namesake of the most recent extension which grants 95 years to corporations and 70 posthumous years to individuals: keeps the flowers fresh.
The Sonny Bono Copyright Term Extension Act also clarifies just what is a not a public performance and therefore fair use: a living room less than 3500 square feet, or if larger than 3500 square feet the TV less than 55" diagonal, and no more than two TVs, and no more than 6 speakers in addition to the speakers built into the TV. It is not clear if woofers and tweeters are counted separately or together (is it drivers or cabinets or channels, Sonny? Get out the Ouija board).
This sort of thing is driven by Disney and other corporations holding valuable intellectual property, mostly in the entertainment industry. Their justifications are pretty lame, they can't just say "we wanna make more money" since that's pretty clearly not the only criterion for copyright. They come up with lame excuses like Marc Gershwin whining "someone could turn `Porgy and Bess' into rap music." As if Porgy and Bess didn't come from African American musical traditions to begin with.
Clearly copyright law is insanely complex, and getting more so. The basic text of the law is unclear and years of contradictory rulings have done little to clarify it. New bills, in particular the No Electronic Theft Act (NET) and the World Intellectual Property Organization (WIPO) acts in the house and senate (HR 2281 and S 2037) really screw it up.
WIPO is an international body that has US representation and is in the process of defining uniform or at least non-conflicting intellectual property law among it's member nations.
Part of the reason for bothering to worry about this stuff, aside from avoiding a jail term, is that HR2281 in particular is a live document and, amazingly, the version released on July 22 addresses some of the concerns of civil liberties lobbying groups like the DFC, EFF, the ACLU, EPIC, and the ALA. If you really want a safe harbor for yourself, you just going to have to get out your checkbook, but there has been some progress in the legislation to make it slightly less heinous.