Public domain

Public domain is not really a license, it's the lack of applicable copyright. It can be viewed as a permission for everyone to do anything with a work, without copyright restrictions. Usually a work is not released into public domain, but if the copyright runs out (depending on the country of origin) it's in the public domain. A work ineligible for copyright is also in the public domain.

A work released in this way grants all the four freedoms listed in the definition of free cultural works:

	The freedom to use and perform the work: The licensee must be allowed to make any use, private or public, of the work. For kinds of works where it is relevant, this freedom should include all derived uses ("related rights") such as performing or interpreting the work. There must be no exception regarding, for example, political or religious considerations.
	The freedom to study the work and apply the information: The licensee must be allowed to examine the work and to use the knowledge gained from the work in any way. The license may not, for example, restrict "reverse engineering".
	The freedom to redistribute copies: Copies may be sold, swapped or given away for free, as part of a larger work, a collection, or independently. There must be no limit on the amount of information that can be copied. There must also not be any limit on who can copy the information or on where the information can be copied.
	The freedom to distribute derivative works: In order to give everyone the ability to improve upon a work, the license must not limit the freedom to distribute a modified version (or, for physical works, a work somehow derived from the original), regardless of the intent and purpose of such modifications. However, some restrictions may be applied to protect these essential freedoms or the attribution of authors.