Since the United States ratified the Berne Convention in 1989, copyright has been automatically granted to authors. For works created after 1978, this government-granted monopoly will last until 70 years after the author's death (or 95 years after publication for anonymous works, pseudonymous works, or works made for hire). This means that, without a license, it is often illegal to reproduce, create derivative works, distribute, publicly perform, or publicly display a modern work, although some uses may be found legal under the doctrine of fair use. Beginning with the movement for software libre in the 1980's, a number of copyright licenses have been written to generally allow for a freer flow of information, by imposing fewer or more targeted restrictions than the law's default set.
Types of Licenses
There are two broad categories of licenses, copyleft, sometimes described as viral, and permissive. Copyleft licenses require that derivative works be made available under the same terms as the original work, while permissive licenses have minimal requirements on derivative works. Licenses have been written for a variety of material in mind, including documentation, art, software, databases (mainly targeting European laws), and hardware.