Open Educational Resources (OERs) are materials that are in the public domain or have been created with open licenses (Creative Commons) rather than protected under standard copyright rules and that are freely accessible and can reused and remixed legally. OER materials are usually digital and can be text, video, images, assessment instruments, instructors' guides, software, and more.
"Why Open Educational Resources Matter" by Brendan Walsh
The Five Principles for OER
David Wiley on the 5 Rs of Open Educational Resources