Last reviewed: 11/2018
Copyright refers to a set of laws that protect original works of authorship or creatorship. Copyright covers both published and unpublished works.
Copyright was established in the United States Constitution, Article I Section 8 "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." Congress has tried to balance the rights of the authors and inventors with the rights of the public since the inception of copyright law.
Copyright protects "original works of authorship fixed in any tangible medium of expression, now known or later developed, from which they can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device." Thus, copyright essentially applies to any original work recorded in some manner.
Copyright applies explicitly to:
musical works, including any accompanying words
dramatic works, including any accompanying music
pantomimes and choreographic works
pictorial, graphic, and sculptural works
motion pictures and other audiovisual works
Several of these categories are interpreted very broadly. For example, literary works includes computer programs, as well as more traditional literary fare. Compilations may also be protected by copyright.
Copyright does not apply 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." While not covered by copyright, these categories may well be protected by other areas of law such as patent law.
Under the current US Copyright Law, copyrights on published works last for 70 years after the death of the author or creator. That has not always been the case though, so determining whether something is still covered by copyright can be cumbersome. If the copyright term has expired, then the item has entered the public domain. Many factors contribute to the length of copyright terms such as when and where the item was published or created, whether it was published with a copyright notice or not, whether the copyright was renewed, and many other factors.
An excellent chart that outlines when materials have or will pass into the public domain can be found at Cornell's Copyright Term website. For an interactive chart, see the Digital Copyright Slider courtesy of the American Library Association Office for Information Technology Policy.
Section 106 of the US Copyright Law gives copyright holders certain exclusive rights, as well as the ability to authorize others to exercise these rights. These rights include the ability to reproduce the copyrighted work, prepare derivative works, distribute the work, and display or perform the work publicly. Sections 107 through 122 establish limitations on these exclusive rights and are very important as they allow many uses that do not require obtaining permissions from the copyright holder. In general, you may use copyrighted works either by determining your use falls within one of these limits such as fair use, or by obtaining permission (including licensing mechanisms) from the copyright holder.