The Public Domain refers to works that have no copyright protection. In other words, works in the public domain can be used without needing to ask anyone for permission to use them. Works that fall into this category include those whose copyright has expired (generally anything created before 1923) and those that never had copyright protection, like most works created by the U.S. Federal Government.
Works fall into the public domain for three main reasons:
1. the term of copyright for the work has expired
2. the author failed to satisfy statutory formalities to perfect the copyright (i.e., did not renew copyright)
3. the work is a work of the U.S. Government.
As a general rule, most works enter the public domain because of old age. This includes works published in the United States before 1923, but may include works published after that date if the copyright was not registered or renewed.
Many works that are in the public domain can be found on the web. (Search for "public domain" amd "public domain images".)