In industrialized nations, a number of rights are typically recognized as applying to the owner of "intellectual property" or "IP". IP encompasses most creative work, such as written or spoken word, images, music, and combinations such as those seen in motion pictures. Typically, where the tangible product of the IP is its printed form, the owner of the IP has a "copyright"; they have the right to assert creative credit, to make and distribute copies of their work, and to prevent anyone else from doing the same. Typically, these rights are "reserved" by the copyright holder; they claim these rights and prohibit anyone else from performing these actions without permission from and consideration to the copyright holder.
With a creative work identified as in the "public domain", one of two things has happened; either the work was created in a time before copyright was recognized by law (in the U.S., this is prior to 1937), or the owner of the IP has relenquished their rights to the work, either explicitly by statement or implicitly by an overt act indicating that they do not wish to exercise their rights as IP holder. Once a work has entered the public domain, a copyright cannot be reclaimed.
Public domain works thus have no intellectual property rights attached to them; they may be freely viewed, copied, distributed, derived from, performed in public, etc, and no consideration is required to be given to the original creator of the work. One right of the creator of the work - the right to be recognized as its creator - is typically recognized even for public domain works, however this attribution is primarily a courtesy extended by those who redistribute the work; there is no legal requirement of attribution of a public-domain work, and therefore no legal penalty for mis-attributing or omitting attribution.