NOTE: The term "software" on this page might, in some circumstances, be just as well interpreted as "product" or "creation" or "work"; as, for examples, software documentation or web pages.
NOTE: Be mindful that the following was written by a non-lawyer.
Public Domain software is software for which nobody owns a copyright. It has been forever freed of all copyright ownership by one of two methods: 1) The government removes ownership after a certain term. 2) The copyright owners voluntarily cede ownership, usually documented by a published statement. Example: "This software and documentation is hereby released to the Public Domain by its copyright owner, John Doe." (Some like to say that the public then owns the copyright, but that is debatable, if not meaningless.)
Anyone can make a minor change to PD software and copyright that, but doing so does not prevent anyone else from using or modifying the PD copy. Placement of a release to the PD in a software licence renders the licence meaningless since some copyrights must be retained to restrict use of the product to licencees. Since most people prefer to restrict use of software to those that agree to not sue the owner and agree to retain ownership and authorship acknowledgements in derivatives, most software is not released to the public domain.
The liability clauses of software licences are so universal they should be written into law so the last important reason to licence software is removed. Many would not insist on acknowledgements or exert other control over the software if they could be free of licencing concerns and their attendent lawyers.
The Free Software Foundation (FSF) makes much use of the words "free" and "freedom" and wants us to believe that products released under its "GPL" are "free". One need only observe the large number of words in that licence (and the LGPL) to realize that the FSF doesn't understand "free". As I read it, one can't publish a computer program in which some of the code is under GPL and the rest has been released to the public domain or any licence other than GPL. Use of the GPL has the insidious effect of forcing all future users of GPL'ed products to use the GPL and not licences which permit more freedom of usage. For example, a subroutine to change the contrast of an image will need to be rewritten from scratch without benefit of the existing code to be used in a graphics program that is mostly written under a different licence, whether that be severerly restrictive, extremely liberal, or public domain. Whenever practical, software should be released under a licence without the GPL's "virus" attributes.