*****Disclaimer: This is not legal advice and is for educational purposes only. This does not create an attorney-client privilege.
Public domain refers to works that are not protected by copyright. In other words, these creative works are free for you to use without having to seek approval from the creator and/or owner of that work. Something may be in the public domain because the intellectual property rights have been forfeited, have lapsed, or are not applicable to the specific circumstances.
Many works that are beloved today are examples of literature, music, art and more that have fallen into the public domain. For example, the work of Michelangelo, Mozart and Shakespeare are all within the public domain. Copyrights are meant to protect the intellectual property of creators and their estates, so that they can control and benefit from the fruit of their own labors. However, things fall into the public domain after a time, once the creators have gone and passed, so that the public and culture can benefit from, sample from, and interpret it. It also protects these works from anyone ever owning them again; they are ideas of the people and culture.
Normally, work enters the public domain because of their old age. Within the United States, generally works created prior to 1923, or works without renewed copyrights that were published before 1964, fall within the public domain. For works that were created and published prior to 1978, copyright renewal was a requirement.
Some works may have fallen into the public domain for other reasons. For example, some works were published without ever notice for copyright, which was a requirement until March 1, 1989. Thus if something was published before this date and they did not file for their copyright, it would also be considered public domain. A work can also be in this class for public use if they creator or owner of the rights to it expressly donated it to the public, forgoing their right to copyright protection (which you no longer have to apply for to receive).
Certain work is inherently of the public domain; chiefly, works created by the U.S. Government. Works created by the US, as well as certain other governments and government entities, are excluded from copyright protections. The kinds of governments works that could fall into this category are enacted laws and enacted building codes, for example. The exceptions that may fall under copyright protection can in some cases include work done by third party contractors, for the United States government.
Sources: