A work of authorship is in the “public domain” if it is no longer under copyright protection, or if it failed to meet the requirements for copyright protection. Works in the public domain may be freely used by anybody without seeking permission from the creator of the work.
There are generally four categories of works in the public domain:
Works created by the federal government (§ 105)
Works explicitly dedicated to the public domain by their authors
Works that copyright does not protect
Works for which copyright protection has expired
Works created by any federal government agency, or through a contract with the federal government, are not eligible for copyright protection and may be freely used.
Works created by state or local government agencies are eligible for copyright protection.
If the author of a work explicitly dedicates the work to the public domain, it may be freely used. If there is no explicit statement on a work to this effect, do not assume that it is in the public domain.
A Creative Commons license or similar open access license may be used to allow for different levels of use by the public. If such a license is used, be sure the proposed use is allowed by the license.
There is no copyright protection for facts or ideas – but there is protection for the way those facts and ideas are expressed! (§ 102). (If, however, there are a limited number of ways to express a fact or idea, that expression is not protected by copyright. For example, you could not copyright the exact phrase "the freezing point of water is 32 degrees Fahrenheit" - there are only a limited number of ways to express that fact).
The term for which copyright protection is valid depends on a number of factors, including when and if the work was published and who the author(s) may be. For a detailed explanation of when certain copyright terms expire, visit here.