The term Open Content describes a creative work (such as a text, images, sound recordings and videos) is published under a licence that explicitly gives permission to copy, adapt or spread the work on certain conditions.
The biggest advantage of using Open Content material is that you cannot, by accident or on purpose, violate copyrights.
The creator of the original material often offers his or her material under a Creative Commons licence in which they state the conditions under which the material may or may not be used. Please note that the rules for citing sources also apply to Open Content material.
One specific category of Open Content material are audiovisual extracts. See Remixables for an overview of sources.