"The foundation of Open Education is Open Educational Resources (OER), which are teaching, learning, and research resources that are free of cost and access barriers, and which also carry legal permission for open use. Generally, this permission is granted by use of an open license (for example, Creative Commons licenses) which allows anyone to freely use, adapt, and share the resource - anytime, anywhere. 'Open' permissions are typically defined in terms of the '5Rs': users are free to Retain, Reuse, Revise, Remix, and Redistribute these educational materials." - SPARC
The University of Oklahoma University Libraries has also produced a very helpful FAQ with more information about OER.
There are a lot of things out there that are free, but that does not make them OER. Free does not equal open. To be considered OER, the material must have an open license. Open licenses enable what are called the 5R permissions: