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 pirating 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 characters and story line of a movie, though, might be considered copyrightable expression by some courts, but not by others. From US copyright law [17 US 102]:
In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
Facts are considered "discoveries" under this law, and are not copyrightable, but in the case of Feist v. Rural, the Supreme Court further detailed this distinction by ruling that the "creative act" may include not only expression, but also selection (deciding which things to include or exclude), and presentation (how they are shown and in what order), so a collection of facts might be able to copyright those creative aspects. (The principle contained in this decision of the U.S. Supreme Court has been incorporated into the WIPO Copyright Treaty, and thus is part of the law of countries other than the United States also.) However, the Feist decision also ruled that a non-selective collection of facts ordered in a non-creative way was not subject to copyright, no matter how much work was necessary to create that collection. This "sweat of the brow" doctrine applies to "white pages" listings in phone books (the source of the Feist dispute) and similar fact collections.
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.
Recent changes in United States copyright law: