The public domain consists of any work that is not protected by copyright. Here are some examples of works unprotected by copyright:
In addition to entire works residing in the public domain, the following portions of protected works are not covered by copyright and thus reside in the public domain:
The arrangement/compilation of facts/data may be protected by copyright if it is minimally creative, and the actual creative expression of the above elements may be protected by copyright, but the above elements themselves are unprotected, and thus reside in the public domain.
The importance of the public domain cannot be overstated. Its existence allows any work contained therein to be exploited in any way without permission by the author. Think of all the works that have been derived from Shakespeare's plays, for example, that were created without permission or license fees, all because Shakespeare's work is in the public domain.