The copyright protection terms are typically described as the life of the author plus a certain number of years after his/her death (or pma: post mortem auctoris). In many countries, including the USA and European Union member countries, the copyright terms expire 70 years pma, while in other countries, including Canada and New Zealand, they last "just" 50 years pma.
When those terms eventually expire and a work enters the public domain, it becomes fully available so that everyone - without any need for prior authorization - can access and use it for any purpose whatsoever.
How important is the public domain for our societies? In prof. James Boyle's words:
"Our markets, our democracy, our science, our traditions of free speech, and our art all depend more heavily on a Public Domain of freely available material than they do on the informational material that is covered by property rights. The Public Domain is not some gummy residue left behind when all the good stuff has been covered by property law. The Public Domain is the place we quarry the building blocks of our culture. It is, in fact, the majority of our culture."
(The Public Domain: Enclosing the Commons of the Mind, Yale University Press, p.40f, 2008).
This Public Domain Day (each year's January 1st) website is an initiative of COMMUNIA, the European Thematic Network on the Digital Public Domain. Our aim is to raise worldwide awareness about the role of the public domain in our societies and to provide resources and information.