Open Educational Resources (OER) "are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others. Open education resources include, full courses, course materials, modules, textbooks, streaming videos, tests, software, and any other tools, materials, or techniques used to support access to knowledge." --The William and Flora Hewlett Foundation
In short, OER are any materials you can use in your teaching and learning that the authors have designated open for free public use.
To be considered a "true" open resource, an OER must be released with a license that allows users to interact with the material in five ways - what we call the Five Rs.
OER allow users to:
Read more about OER: