OER are freely available teaching and learning materials that are in the public domain or are openly licensed to allow everyone access to:
Retain- ability to own, keep, and make copies of the item
Revise- ability to edit or modify the content
Reuse- ability to use the content in different ways
Remix- ability to combine content with other material
Redistribute- ability to share content with others
OER can be textbooks, videos, online courses, or any other format.
Note: the easiest way to identify OER is to look for a Creative Commons License or a Public Domain mark.