Wikipedia: Public domain

The body of creative works and other knowledge--writing, artwork, music, science, inventions, and others--in which no person or organization has any proprietary interest (particularly a [government-granted monopoly]? such as a copyright or patent) is called the public domain. Such works and inventions are considered part of the public's cultural heritage, and anyone can use and build upon them without restriction (not taking into account laws concerning safety, export, etc). A work or invention becomes part of the public domain in one of several ways:

Absence of legal protection

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. 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 Aristotle.

Expiration

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 work was created before January 1, 1923, or at least 95 years before January 1 of the current year, whichever is later.
The last surviving author died at least 70 years before January 1 of the current year.
No Berne Convention signatory has passed a perpetual copyright on the work.
Neither the United States nor the European Union has passed a copyright term extension since these conditions were last updated. (This must be a condition because the exact numbers in the other conditions depend on the state of the law at any given moment.)

These conditions are based on the intersection of United States and European Union copyright law, which most other Berne Convention signatories recognize. Note that copyright term extension under U.S. tradition does not restore copyright to PD works (hence the 1923 date), but European tradition does. Note further that works created by a United States government agency fall into PD at the moment of creation.

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 routinely provides millions of U.S. dollars of campaign money to legislators in exchange for copyright term extensions.

Disclaimer of interest

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]?.

Ineligibility

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.

Licensing

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/