What is Public Domain Software?
Public Domain Software (PDS) refers to software that is not copyrighted and that anyone can use for free without restrictions. PDS does not include product promotions of free software, "free ware," or "shareware" all of which have use restrictions and copyrights.
Public domain software does not necessarily allow a user to access, use, or alter source codes, but in some cases, it may.
Open Source Applications do Not Fall Under Public Domain
The key distinction between Open Source (OS) and public domain software is not whether or not source code is accessible, but if there are any licensing requirements or other restrictions on use, altering the source code, redistribution, or copyrights. If there are, it is Open Source - not public domain software.
Open Source Initiative, a 501(c)(3) California-based nonprofit, offers a very detailed (and legal) definition of open source software and how and who can use it.
They also have a great alphabetical listing of companies that offer open source software if you wish to investigate a particular company.
If there are absolutely no restrictions or copyrights on software applications, it is considered public domain software.
The Free Software Foundation offers a comprehensive list of free software.
