Work that is in the public domain includes any work for which the copyright has expired, and not been renewed, any work that was published prior to 1923, works produced by the United States Government, and other specific Governments, Work that is donated to the public domain, or any other work that has no laws that restrict its use by the public.

The work itself may be in the form of a book, an article, a song, a film, a photograph, a painting, or even an invention. These works that are not covered by copyright laws, or are no longer covered by copyright laws, are free for the public to use as they see fit.

This does not mean that if a work is freely obtained it is in the public domain. In other words, if you saw it on a website, for free, that doesn't mean that it is okay for you to use it. In fact, most things that are written down, in any form, are automatically copyrighted.

Public domain refers to work in which the copyright has run its course. According to the Copyright Office of the United States:

“The term of copyright for a particular work depends on several factors, including whether it has been published, and, if so, the date of first publication. As a general rule, for works created after January 1, 1978, copyright protection lasts for the life of the author plus an additional 70 years. For an anonymous work, a pseudonymous work, or a work made for hire, the copyright endures for a term of 95 years from the year of its first publication or a term of 120 years from the year of its creation, whichever expires first. For works first published prior to 1978, the term will vary depending on several factors.”

Public domain also refers to work that was meant for the public domain, such as work that is donated, as well as work that never had a copyright, which would be work that was created prior to 1923, or work that was not copyrightable material.

These materials are free for the public to use, in any manner that they choose. You can use bits and pieces of a public domain work, or the entire work itself. You can create new work from the bits and pieces, or sell an entire work – as you see fit.