The public domain is a state which describes works not protected under copyright laws. Works in public domain may be freely used in any way without any limitations.
Copyright terms and the public domainEdit
Both the Statute of Anne (the original UK copyright law) and the US Constitution (which, in Section 8, allowed the Congress to enforce copyright laws) mentioned that the works are only covered for a limited term -- after which they were supposed to go into the public domain. Nowadays, a result of authors' and publishers' requests, the terms in many countries were greatly extended to the point that, in the United States, no works will enter public domain by expiration until 2019.
Quite often, works still under copyright law may become completely unavailable to the public -- for example, when the copyright owners can't even be found -- in that case, these works may be called "orphan works". That makes it impossible to get access to such works legally. Duke University's School of Law's Center for the Study of the Public Domain claims that, for works released between 55 and 75 years ago, only 2% of them continue to hold commercial value.
Willful public domain dedicationEdit
In some cases, authors or publishers may willfully waive their rights and submit their works into public domain. While, technically, all one has to do for that (at least in the United States) is to make a statement about releasing all rights to the work, some claim this has no actual legal effect. And in some other countries (for example, in Germany) it is actually impossible to do so at all.
An alternative solution that works in the whole world is to release a work under a license that gives the public all the actual rights. While the original work will still, technically, be covered by copyright, everybody else will share the same rights as if the work was in the public domain. Examples of such licenses are Creative Commons' CC0 and the WTFPL.