Warning! Many people use the term "public domain" incorrectly to describe material that is "available to the public."
Some people even say their work is in the "public domain" and then try to put restrictions on it. For example, they may say it is in the public domain as long as it is used for educational or noncommercial purposes. Such a statement is legal nonsense (and there's no telling how it might be interpreted in court). If you want restrictions, then a better statement would be to say that it is copyrighted by the appropriate author, who will automatically grant a nonexclusive, royalty-free license to anyone who wants to use it for educational or noncommercial purposes. Alternatively, if you really do want to put your work in the public domain, then you must do so without any restrictions.
Warning! Once a piece of intellectual property moves into the public domain, that status can never be revoked.
One reason this could be important is because, if the original work were covered under copyright, then legitimate adaptations and derivatives are also covered by copyright. On the other hand, if the original is in the public domain, then their authors have no recourse if someone else chooses to use them in ways that they consider objectionable.