= The public domain is a very simple concept - no rights are reserved to owners, and all rights are granted to users. The public domain exists as a counterweight to copyright in the creative space 
Public domain is defined as a body of intellectual endeavours unfettered by law. 
In the strict sense of copyright law, the "public domain" refers to those works which are no longer or have never been protected by copyright. 
It's also the title of a book by James Boyle.
Definition and problematic of the public domain: “The public domain is a space where intellectual property protection does not apply. When copyrights and patents expire, innovations and creative works fall into the public domain. They may then be used by anyone without permission and without the payment of a licensing fee. Publicly owned national parks are also considered by many to be public domain lands. Because of the extensions of the terms of both copyrights and patents, and the privatization of lands and other resources owned by the Federal Government, little is now entering the public domain. Since the public domain is a treasure trove of information and resources to be used by future generations, many advocates are concerned that its stagnation will make it more difficult for future generations to find creative inspiration." (http://www.centerpd.org/public_domain.htm)
From the Wikipedia:
"Public domain comprises the body of knowledge and innovation (especially creative works such as writing, art, music, and inventions) in relation to which no person or other legal entity can establish or maintain proprietary interests within a particular legal jurisdiction. This body of information and creativity is considered to be part of a common cultural and intellectual heritage, which, in general, anyone may use or exploit, whether for commercial or non-commercial purposes." (http://en.wikipedia.org/wiki/Public_domain)
"The public domain is a category of works made up of text, images and documents that are not protected by an active legal copyright. On the Internet, material in the public domain can be freely downloaded, copied and reused.
Creative material usually ends up in the public domain in one of two ways. Some creators relinquish their copyright and donate their work to the public domain.
Works can also fall into the public domain if their copyright has expired. In general, books published before 1923 in the United States are considered in the public domain." (http://communia-project.eu/node/180)
The public domain and the Commons
"If public domain can be considered a container, then the commons represents its content of inexhaustible resources, jointly held and accessible without permission." 
Most of the time, both concepts are used interchangeably, though the Commons seems to overtake the Public Domain in terms of popularity. The public domain concept relates the 'outside' of the intellectual propery system, i.e. items without copyright, and thus stresses the open access features: nobody can be excluded. The Commons stresses the absense of state, corporate and individual control, in favour of distributed control, and is related to non-private and non-state common property regimes.
For an investigation of the differences between the concepts, see the essays by James Boyle, at http://www.law.duke.edu/pd/papers/boyle.pdf (The Second Enclosure Movement and the Construction of the Public Domain) and http://www.law.duke.edu/boylesite/foreword.pdf (The Opposite of Property)
The Public Domain and U.S. Copyright law
Wikipedia states that
"Robert A. Baron argues in his essay “Making the Public Domain Public” that “because the public domain is not a legally sanctioned entity,” a statement disclaiming a copyright or “granting” a work into the public domain has no legal effect whatsoever, and that the owner still retains all rights to the work not otherwise released. The owner would then have the legal right to prosecute people who use the work under the impression that it was in the public domain. It is certainly true that under some jurisdictions (which ones?), it is impossible to release moral rights, though that is not the case in the United States. A more likely problem may be the lack of factual evidence that the owner has indeed put the work into the public domain.
Some scholars of copyright law, including Lawrence Lessig, agree that it is difficult to put works in the public domain, but not impossible. The Creative Commons website, for example, has a public domain dedication form which produces an electronic receipt which is meant to act as legal backing for the dedication. Even if it is ruled that a work cannot be released into the public domain, a thorough dedication such as this one also releases all rights, so that the author retains only a free-use license. Lessig, however, argues that another licensing option, such as the Creative Commons Attribution-Only license, is a safer choice, and that click-through agreements are insufficient to put works in the public domain." (http://en.wikipedia.org/wiki/Wikipedia:Granting_work_into_the_public_domain)
St. Peter explains what needs to be done at https://stpeter.im/?p=1780:
"1.American law was modified in 1976 to automatically put works under copyright, thus (seemingly) removing the option to place one’s works directly into the public domain. This change supposedly protected creative individuals but in fact restricted their freedom.
2. The identity of the author is not necessarily clear from a legal perspective. Yes, I own saint-andre.com (look it up in whois), so presumably I have control over what is posted at that domain. But is my assertion that I am the author of, say, Who’s Afraid of the Public Domain? legally binding? Well, it appeared at my website first, but it’s not digitally signed, so who knows.
To overcome some of these challenges, Creative Commons provides a public domain dedication service whereby the author enters an author name, email address, and work title into a web form and Creative Commons sends an electronic receipt to the author and to Creative Commons. But who says that a given email address can be reliably associated with the author? A stronger method would involve the use of digital signatures, but that is prohibitively difficult for the average creative individual, who doesn’t know what a digital certificate is, let alone have the tools that would make it easy to digitally sign documents, images, and the like — heck, even I don’t know how to digitally sign an HTML file with OpenSSL, and (thanks to Thunderbird) I digitally sign the email messages I send!
It’s clear to me that we need better tools to enable rights-assignment (including assignment to the public domain) during the authorship phase — support in word processors, desktop publishing programs, image creation applications, music recording software, and so on. We need better ways to associate electronic files with authors, whether through digital signatures or some other means. We need ways to register public domain works with a neutral third party such as Creative Commons. Then we need to start testing these mechanisms among authors and in the courts of law and public opinion."
Amy Kapczynski on the concept of the Public Domain in the A2K Movement
By Amy Kapczynski:
“The concept of the public domain is central to the new politics of A2K, although
not, as we will later see, always uncontroversially so. It is drawn from judicial and
legal discourse, where it has long been used to refer to informational works that
are not covered by intellectual property law, for example, because the copyright or
patent term has expired.50 In the 1980s and 1990s, scholars critical of the expansion
of intellectual property rights seized upon the term to carve out a positive iden-
tity for the “outside” of intellectual property.51 As James Boyle put it, “The envi-
ronmentalists helped us to see the world differently, to see that there was such a
thing as ‘the environment’ rather than just my pond, your forest, his canal. We
need to do the same thing in the information environment. We have to ‘invent’ the
public domain before we can save it.”52 Key here was early work of David Lange,
who argued that no intellectual property right “should ever have affirmative rec-
ognition unless its conceptual opposite is also recognized. Each right ought to be
marked off clearly against the public domain.”
Lange’s early articulation of the term marks the abiding influence of intellec-
tual property law on the concept of the public domain. The public domain here is
defined as the “conceptual opposite” of the domain of exclusion rights protected
by intellectual property. The same relationship is emphasized in James Boyle’s def-
inition of the public domain as “material that is not covered by intellectual prop-
erty rights” as well as “reserved spaces of freedom inside intellectual property.”54
In the simplest sense, then, A2K advocates use the term positively, to bring
into focus the negative space of intellectual property law and to articulate its
importance for innovation and creativity. The public domain thus becomes not just
the opposite of intellectual property, but also an essential—and endangered—
component of our creative and informational ecology. Included herein are not just
older works in the literary or technical arts, but also resources such as language
and scientific theories that are free of intellectual property rights and to which we
have a common right. Many of these resources were never protected as intellectual
property at all, thus demonstrating that private rights are not necessary to the pro-
duction of all informational goods. Such goods and the ability to use them freely
are also clearly central to our ability to think and create. The emphasis on the pub-
lic domain thus is used to counter “the romantic idea of creativity that needs no
raw material from which to build” that characterizes the despotic dominion theory
of intellectual property and to call attention to the need of every creator to have
access to the scientific or cultural domain that precedes and surrounds her. Boyle,
for example, contends that the “public domain is the place we quarry the building
blocks of our culture. It is, in fact, the majority of our culture.”55
The A2K movement calls upon the public domain in this way to make the case
that the account offered by the despotic dominion theory of intellectual property
is radically incomplete as a description of both the world as it is and the world as
it should be. Even now, in the most absolutist period of intellectual property law
we have known, our creative world remains largely beyond the reach of intellec-
tual property rights. And intellectual property rights as we know them bear little
resemblance to property rights over material resources, with far greater freedoms
reserved for nonowners. If so-called “real property” rights worked like copyrights,
for example, the home you built would be turned over to the public some fifty
to seventy years after your death. In the meantime, if others wanted to use your
front porch to criticize you, you would have to permit it.56 It turns out that ideas
are different from material goods and are treated as such by the law. The concept
of the public domain calls attention to this fact—a fact that the despotic dominion
account papers over.
The concept of the public domain calls the despotic dominion account of intel-
lectual property into question in yet another way, by emphasizing the “public”
values that a public domain serves—and that the privatization of intellectual cre-
ations threatens. This is the public domain as opposed to the private domain—the
domain that the despotic dominion conception of intellectual property equates
with the public good.
We can begin by asking what is “public” about the public domain. Is it public
like a public park? Like public assistance? Like the public good? Like a public fig-
ure? A2K narratives about the public domain treat what is public as synonymous
with what is “open to all,” but in two different dimensions: that of permission and
that of price.
Public-domain material is presented as important to our creative ecology, on
the one hand, because one need not ask permission to use it—which is to say,
no one has the legal privilege to deny another the ability to use it. If you want to
rewrite a Jane Austen novel, retaining most of her words, but inserting zombies,
no representative of Austen’s estate can deny you permission, because the work
is now in the public domain.57 A2K advocates thus celebrate the public domain as
a place free of the political control or personal caprice of others. This is contrasted
with the world of intellectual property, where owners of works may stop others
from using their creations in ways of which they disapprove.58 When DJ Danger
Mouse became an overnight sensation for an album remixing the Beatles and Jay-Z,
for example, he also earned the attention of lawyers for the Beatles’s label, who
forced him to stop distributing the album. Copyright facilitates consolidated con-
trol and disrupts semiotic recoding. The need to obtain permission, A2K advocates
argue, is thus in tension with the desire for an open and democratic culture.59
The public domain is “public” in another sense. Like a public street, it may be
traversed and used by all comers without individualized permission. But also like
a public street (if not necessarily a public highway), it may be traversed without
payment. (In the phraseology of Richard Stallman of free software fame, it is both
“free as in speech” and “free as in beer.”) No one pays for what they take from the
public domain (there is no licensing fee), so works available in the public domain
are available, in theory, at or close to their marginal cost of distribution—the
cost of printing and selling a book, for example, without an additional fee for the
author who wrote it. And of course, in a world of pervasive digital networks, the
cost of distribution indeed moves toward zero, meaning that works out of copy-
right may be available for no cost at all. The public domain thus has a differential
value for those who have limited financial means. In this sense, it is public in the
way that public assistance is public—it represents a kind of state subsidy for those
who cannot afford the licensing fees and lawyering costs associated with private
markets in information.”
(Source, from the introduction: Access to Knowledge: A Conceptual Genealogy. By Amy Kapczynski. In the book: Access to Knowledge in the Age of Intellectual Property) (https://stpeter.im/?p=1780)
Public Domain advocacy organizations
The Center for the Public Domain (U.S.), is at http://www.centerpd.org/
Public Knowledge (U.S.), is at http://www.publicknowledge.org/
The Union for the Public Domain (international), is at http://www.public-domain.org/
- James Boyle. The Public Domain: Enclosing the Commons of the Mind.
Review from Felix Pleşoianu:
"an incursion into the many (and often ugly) issues surrounding copyright and patents, since their invention during the 18th century and up to 2007. With exceptionally well-documented case studies and well-thought-out arguments, the author makes the case for striking a balance between the rights of authors and those of the public at large.
And that's my big gripe with the book: throughout the 300+ pages, Mr. Boyle keeps repeating that intellectual property laws are ultimately good. But every single example he gives demonstrates how creativity flourishes in the absence of (and often despite) legal "protections", how these legal protections keep expading in scope and duration at the whim of business lobbysts, and how the only real hope nowadays appears to come from initiatives such as the GPL and Creative Commons, which turn copyright on its head.
Oh, the book does bring up (in passing) the tired old example of pharmaceutical research, which supposedly would not be done in the absence of patents, as it is too expensive. But this one doesn't hold water, as the author himself admits when he suggests (again, in passing), that some of this research would be made cheaper by a simple sharing of efforts among the companies involved.
For the most part, though, the book is plain great. I had no idea that a single song can have such a rich history. Or that copyright has stirred so much controversy since the very beginning. It does become a little difficult to follow halfway through - as the preface warns it may happen - but the last 40% or so more than make up for it." (http://my.opera.com/claudeb/blog/2009/01/06/digital-week-6-a-comedy-of-the-commons)
- Faq on the public domain, at http://www.sarai.net/publications/readers/01-the-public-domain/016-023faq.pdf
- The Creative Commons public domain license is at http://creativecommons.org/licenses/publicdomain/
- Communia: The European Thematic Network on the Digital Public Domain
- Bibliography on the public domain at http://homepages.law.asu.edu/~dkarjala/OpposingCopyrightExtension/publicdomain.html
- Interview: James Boyle on the Endangered Public Domain
- How big would the public domain be without term extensions?, see: http://www.rufuspollock.org/2010/05/26/the-size-of-the-public-domain-without-term-extensions/