The public domain is made up of works on which copyright has expired, material for which the author never claimed copyright, and material produced by the federal government. The majority of works in the public domain are there because they are old. Examples of public domain works include the stories of Edgar Allen Poe, the poetry of Emily Dickinson, the works of Herman Melville, and newspapers from the Civil War. Just be aware that adapted uses of public domain works may still fall under copyright. The novel Moby Dick is public domain; the motion picture is not.
Public domain material is freely available for anyone to use. For example, you could make a graphic novel based on a story by Poe without having to ask permission--but so could anyone else. You can make copies of public domain material, post it on websites, use
it in your own work--while still giving proper citation and credit to
the author, of course. You may not have to worry about copyright
infringement, but you do not want to plagiarize.
Most works published in the U.S. before 1923 are public domain. Some works published between 1923 and 1964 are also in the public domain if their copyrights weren't renewed. Back in the day, the term of copyright was shorter and renewal was a possibility. See Copyright Term and the Public Domain in the United States by Peter Hirtle, of Cornell University, for the full details about when various works enter the public domain.