While copyright was created to protect the financial incentive of those doing creative work as a means to encourage more creative work, works in the public domain just exist as such, and the public have the right to use and reuse the creative work of others without financial or social burden.
Without some kind of grant of monopoly rights -- so-called "intellectual property rights" -- all works belong to the public domain. When copyright or other protections reach the end of their life, works are said to revert to the public domain.
Creative works are in the public domain wherever no law exists to establish proprietary rights, or where the subject matter is specifically excluded from existing laws. For example, most mathematical formulas are not subject to copyrights or patents in most of the world (although their application in the form of computer programs can be patented). Likewise, works that were created long before such laws were passed are part of the public domain, such as the works of William Shakespeare and Ludwig van Beethoven and the inventions of Archimedes.
Most copyrights and patents have a finite term; when it expires, the work or invention falls into the public domain. In most of the world, patents expire 20 years after they are filed. Trademarks expire soon after the mark becomes a generic term. Copyrights are more complex; generally, they expire worldwide when all of the following conditions are satisfied:
The situation with respect to British government works is a little more complex, but still relatively easy to understand. British government works are protected by either Crown Copyright or Parliamentary Copyright. Published Crown Copyright works become public domain at the end of the year 50 years after they were published, unless the author of the work held copyright and assigned it to the Crown. In that case, the copyright term is the usual life of author plus 70 years. Unpublished Crown Copyright documents become public domain at the end of the year 125 years after they were first created. Parliamentary Copyright documents become public domain at the end of the year 50 years after they were published. Crown Copyright is waived on some government works provided that certain conditions are met.
These numbers reflect the most recent extensions of copyright in the United States and Europe. Canada and Australia have not at present passed similar twenty-year extensions, meaning that copyrights expire in both places after 75 years or 50 years (instead of 95 or 70, respectively). As a result, works ranging from Peter Pan to the stories of H. P. Lovecraft are public domain in both places.
Examples of inventions whose patents have expired include the inventions of Thomas Edison. Examples of works whose copyrights have expired include the works of Carlo Collodi and most of the works of Mark Twain. Examples of works under a statutory perpetual copyright include many of the Peter Pan works by J. M. Barrie. Note that works of The Walt Disney Company are not under statutory perpetual copyright on paper because the United States Constitution requires copyrights to last "for limited Times" (Article I, section 8, clause 8), but Disney and other large publishers routinely provide millions of U.S. dollars in campaign money to legislators, allegedly in exchange for copyright term extensions.
An author or inventor can explicitly disclaim any proprietary interest in the work, granting it to the public domain. Because copyright applies by default to all works, authors must do this explicitly. On the other hand, publishing the details of an invention before applying for a patent may place an invention in the public domain. For example, once a journal publishes a mathematical formula, it may no longer be used as the core of a claim in a software patent. There is an exception to this, however: in US (not European) law, an inventor may file a patent claim up to one year after publishing it (but not, of course, if someone else published it first).
Laws may make some types of works and inventions ineligible for monopoly; such works immediately enter the public domain upon publication. For example, US copyright law[?] releases all works created by the US government into the public domain, patent law excludes inventions that obviously follow from prior art[?], and agreements that Germany signed at the end of World War I released such trademarks as "aspirin" and "heroin" into the public domain in many areas.
Note that there are many works that are not part of the public domain, but for which the owner of some proprietary rights has chosen not to enforce those rights, or to grant some subset of those rights to the public. See, for example, the Free Software Foundation which creates copyrighted software and licenses it without charge to the public for most uses under a class of license called "copyleft", forbidding only proprietary redistribution. See also Wikipedia, which does much the same thing with its content under the GNU Free Documentation License. Sometimes such work is mistakenly referred to as "public domain" in colloquial speech.
Note also that while some works (especially musical works) may be in the public domain, U.S. law considers transcriptions or performances of those works to be derivative works, potentially subject to their own copyrights. For more details (or for public domain transcriptions of public domain works), visit Mutopia at http://sca.uwaterloo.ca/Mutopia/
"Public access to literature, art, music, and film is esssential to preserving and building on our cultural heritage. Many of the most important works of American culture have drawn upon the creative potential of the public domain. Frank Capra's It's a Wonderful Life is a classic example of a film that did not enjoy popular success until it entered the public domain. Other icons such as Snow White, Pinocchio, Santa Claus, and Uncle Sam grew out of public domain figures." ( (http://www.creativecommons.org))