Generally, intellectual property falls into two broad categories, material in the public domain, for which no copyright protections are afforded, and material under copyright and therefore protected by copyright law. (A third class of materials exists, notably materials under copyright protection but for whom no owner can be determined or located. Such works are referred to as 'orphaned works.') Generally, for works created today, copyright law in the United States offers protection for the life of the author plus 70 years. However the specific protection given to any particular work depends greatly on exactly when the work was published, and when the creator may have died (or whether the creator was a commercial entity).
Cornell University publishes a detailed table of conditions that describe the definition of public domain as specified by United States Copyright Law. Please remember that copyright law varies across international borders, and materials under copyright in the United States may be in the public domain in other countries.
The following tools can also assist in determining whether a particular work is covered under copyright law. You will generally need to know the date of first publication and in some cases the death date of the creator.
For works found to be in the public domain, there are no restrictions or required permissions on publishing, editing, redistributing or otherwise using the material in any context.