Copyright is a form of intellectual property; the protection is "grounded in the U.S. Constitution and granted by law for original works of authorship fixed in a tangible medium of expression." Both published and unpublished works can be protected.
Copyright law advances science and the arts by awarding creators a limited but exclusive set rights; creators can control the reproduction, distribution, display, performance, and adaptation of their work. Thus, they have an economic incentive to create.
When the limited period of protection expires, the work enters the public domain. In the US, Congress continues to extend the limited time period of protection.
Founding Fathers: Limited Term
Originally, US copyright was for 14 years, which could be extended to 28 years if the author was alive. They wanted to prevent a system of "hereditary privilege" like that in Renaisannace Europe where printing guilds held "rights to dead authors and tightly controlled what could or could not be printed and who could or could not use literary material."
According to the National Review (2002):
We now have an exact replica of the medieval Stationers' Company, which controlled the English copyrights, only its names today are Disney, Bertelsmann, and AOL Time Warner. The big media companies, holding the copyrights of dead authors, have said, in effect, that Jefferson, Madison, and Hamilton were wrong and that we should go back to the aristocratic system of hereditary ownership, granting copyrights in perpetuity.
Under the law in effect before 1978, copyright endured for 28 years from the date copyright was secured. Copyright was secured by either publishing with a copyright notice or by registering an unpublished form of the work.
In year 28, the copyright holder could apply for a renewal or extension. The Copyright Act of 1976 extended the renewal term from 28 to 47 years for copyrights that existed on January 1, 1978, or for pre-1978 copyrights restored under the Uruguay Round Agreements Act (URAA), making these works eligible for a total term of protection of 75 years.
In 1992, Congress amended the 1976 Copyright Act, providing for automatic renewals for works secured between January 1, 1964, and December 31, 1977 - Public Law 102-307 (txt).
In 1998, Congress extended the life of copyright another 20 years - Public Law 105-298 (txt). Renewal was 67 years, granting a total term of protection of 95 years. According to the National Review (2002), this law (Sonny Bono Copyright Term Extension Act) also mandated that
... there will be no copyright expirations for 20 years, meaning that everything published between 1923 and 1943 will not be released into the public domain. Presumably they'll take up the matter again in 2018 and decide whether any of these books, movies, or songs are ever set free. There are 400,000 of them.
The US Supreme Court affirmed the extensions provided for in the Bono Act. Decision - PDF
Works Created or Published On or After 1 January 1978
Regardless of creation date, works published after this date have the same protections as those created after this date. A work that is created on or after January 1, 1978, is automatically protected from the moment of its creation for the period of the author's life plus an additional 70 years after the author's death. If there are two or more authors, the period lasts 70 years after the last surviving author's death.
Works for hire or those published anonymously are protected for 95 years from date of publication or 120 years from date of creation, whichever is the shorter period.
Next: What is Protected?