Public domain works or content that isn’t protected by copyright law may not be protected for a variety of reasons, including the following:
The duration of copyright in the work has expired. In the U.S., for example, the copyright in a book expires 70 years after the death of its author. (In Canada, copyright expires 50 years after the author’s death.) The minimum duration of copyright protection as set out in the leading copyright treaty, the Berne Convention, is life-plus-fifty but many countries now have a life-plus-seventy duration as in the U.S.
The work has been produced by the U.S. federal government. In the U.S., works produced by the federal government don’t have copyright protection. However, a work produced by a consultant or freelancer to the government may have protection and that copyright may be transferred to the government. Note that in other countries such as Canada, there is copyright protection in federal government works.
The work isn’t fixed in a tangible form. For example, a work such as a speech, lecture or improvisational comedy routine that hasn’t previously been written or recorded in any manner isn’t protected by copyright and therefore is in the public domain.
The work didn’t include a proper copyright notice prior to 1 March 1989. In the U.S., this doesn’t apply to works created after 1 March 1989 since a copyright notice is no longer mandatory to protect a work. However, prior to that date, notice of copyright was necessary on all published works — without this notice, the work went into the public domain. Note that most countries don’t have a copyright notice requirement.