: I am not a lawyer
That being said, public domain is a term applying to copyright statute, referring to a work whose copyright has expired and passes into public ownership. The tales of Mark Twain and earlier works of Sir Arthur Conan Doyle and Edgar Rice Burroughs, the King James Bible, The Three Musketeers, The Swiss Family Robinson, Shakespeare's plays, and many others are examples of public domain literature. They may be reprinted and resold by others, translated, used as the basis for new books or movies, or have other derivative works created from them. Project Gutenberg and Wiretap have archived many public domain works electronically.
In the United States, most books published before 1923 are now in the public domain. Sadly, it will be years before any books from later time periods enter the public domain, thanks to recent changes in copyright law. Corporate concerns such as Walt Disney and descendants of long-dead authors and artists have gradually eroded the "limited time" clause of copyrights as stated in the United States Constitution from its original twenty-eight years to life of author plus seventy-five...which means that there is little probability that even a work created in your grandparents' generation will pass into the public domain while you are still alive.