It is not at all uncommon for an instructor to comment to us "Well, I only use things in the public domain" and be referring to items that are merely freely available on the web. These are not the same thing. Remember that the vast majority of content on the web, though freely available, is protected by copyright. The copyright term is very long and, because the copyright laws changed several times throughout the 20th century, with changes to the law sometimes being retroactive, it can be very challenging to figure out if a given piece of content created in the 20th century is in the public domain or not.
This chart, Copyright Term and the Public Domain in the United States, provides detailed information about all of the possibilities.
Things that are in the public domain may be used without limitation.
