A work which is not copyrighted is in the public domain, and may be freely copied by anyone. It may have been placed in the public domain by its creator, it may be ineligible for copyright (not original enough or otherwise excluded), or the copyright may have expired: in the United States for example, almost all works published prior to 1923 are public domain because their copyright term expired and in the UK and much of Europe, all musical recordings are in the public domain 50 years after release. (In the United States special laws have been passed to extend copyright for certain works beyond the normal term.) See also List of countries' copyright lengths.

All work produced by employees of the US federal government as part of their work is public domain within the US—thus, much of the content found on US government websites (.gov and .mil) is public domain. However, the government frequently includes works on its websites which are copyrighted by someone else, and the US government can even own copyright on works which are produced by others. In other words, some US Federal websites can include works which are not in the public domain--check the copyright status before assuming something is public domain. Note also that this applies only to the US Federal government. Most state governments retain the copyright on their work (California and Florida being notable exceptions).

Works produced by the UK government are not public domain; they are covered by Crown copyright.

Seeing something on the Internet without a copyright notice does not mean that it is in the public domain.

If public domain work is included in a copyrighted product then the new product is not public domain. The portions of the new copyrighted work that are from a public domain source may be removed and copied without permission. For example when a public domain text is included in a Wikipedia article any additional text or new creative elements are still under CC-BY-SA and the GFDL.