The Congress shall have the 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 U.S. Congress passed the first such copyright law in 1790. The Act gave an author an exclusive right to publish and vend "maps, charts and books" for a period of 14 years, with the right of renewal for another 14 years if the author was still alive. The act did not regulate other kinds of writings, such as music or newspaper and specifically noted that it did not prohibit copying the works of foreign authors. The vast majority of writings were never copyrighted - between 1790 and 1799, of 13,000 titles published in the United States, only 556 were copyrighted.
Copyright law has been modified many times since to encompass new technologies such as music recording, to extend the duration of protection, and to make other changes. US courts have interpreted this clause of the Constitution to say that the ultimate purpose of copyrights is to encourage the production of creative works for the public benefit, and that therefore the interests of the public are primary over the interests of the author when the two conflict. These rulings have since been formalized into fair use laws and decisions.
The distinction between "idea" and "expression" is a fundamental part of US law, but it is not always clear. A paper describing an industrial process is copyrightable; it may not be reproduced by anyone else without the author's permission. But the process itself (which is an idea rather than a specific expression) is not copyrightable, though it may be patentable. Another author is free to describe the same process in his own words without violating copyrights (though he might not be able to use the process if it is patented; the articles on Fractal transform and LZW are examples of this situation). Courts disagree on how much of the story and characters of a copyrighted novel or film are considered copyrightable expression. From US copyright law [17 US 102]:
Sometimes the identification of a work's "author" is unclear, and there are many court rulings applying to those situations as well. For example, work done "for hire", that is, specifically at the direction of an employer who pays for the work, is the property of the employer. In other words, if a company hires a writer to write something specific, the company, not the writer, is considered the "author" of that work and owns the copyrights. Any other work done by that writer on his own without compensation and without using company resources usually is still owned by the writer (though employers often try to claim ownership of such work).
Copyrights may be sold, given, or licensed. For example, an author might license the right to publish a translation of a book (considered a derivative work) to a foreign publisher, charging a fee for the license. However, US copyright allows an author (or certain heirs defined by law) to terminate a copyright transfer during the period from 35 to 40 years after the transfer (for transfers made 1978 or later) or 56 to 61 years after the onset of copyright (for transfers made prior to 1978). An agreement made by the author to contract away his right to terminate the transfer is not enforceable, and neither is an attempt by an author to do so so on behalf of his heirs.
More complete details of US copyright law can be retrieved from the Library of Congress.
Title 17, Section 105, United States Code provides that:
Recent changes in United States copyright law: