According to the definition at Copyright.gov: "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 used freely without the permission of the former copyright owner." For ethical reasons, you should still provide proper attribution even if works are in the public domain. Some providers have specific instructions on attribution verbiage.
Creative Commons licenses give individual creators and large institutions a standardized way to grant the public permission to use their creative work under copyright law. There are six different license types with different levels of permissiveness so you need to determine which license your object falls under. (Creative Common License types)
The lists below will help you find public domain and Creative Commons licensed works, but you are responsible for ascertaining if a particular work is in the public domain or has a Creative Commons license that allows you to reuse it.
After searching for videos on YouTube, you can filter your results by limiting to videos with Creative Commons licenses.
