The Creative Commons allows creators to license their work for use by the public under their own terms. Not all works with a CC license are within the public domain. Make sure to check the license type for how the creator wishes for their work to be used.
"Creative Commons Licenses Infographic" by ricardo56 is licensed under CC BY-SA 2.0.
Here are several websites you can use to find content with a CC license.
Openverse is the Creative Commons search feature. It is a an open source project between the Creative Commons and Wordpress.
Looking for music? Try CC Mixter or Jamendo.
Photos for Class is an easy to search service full of images ready to be used in the classroom or a presentation.
Wikimedia Commons is a volunteer run media file repository of audio, images, and video clips that are in the public domain or are freely licensed education materials. Below is a sample image from Wikimedia.