Open Educational Resources are educational materials and resources that are publicly accessible meaning that they are openly available for anyone to use and under some licenses to re-mix, improve and redistribute.
You may already be familiar with open access journals and books. These materials are not "free". Someone had to create them and costs were borne by some party. But the author or publisher licensed the content so that any member of the public may access it and possibly re-use or re-format it.
OER is similar in that the individuals who create these resources are licensing the content so that it is publicly accessible and may be re-purposed by others for educational applications.
OER is a movement in education that seeks to counter costly, commercially produced learning content, typically textbooks, with publicly accessible content that is licensed so that it can be freely distributed and shared. Here is an official definition from the Organization for Economic Co-operation and Development:
Educators and learners as well as learning institutions are driving its development. OER provides an alternative to the rising costs of education. It alleviates the burden of student debt while providing opportunities to students who might not otherwise be able to afford or access materials. For example, in some countries like South Africa, many educators and learners are tapping into OER as the only source for textbooks.
OER provides an opportunity to try new ways of teaching and learning, many of which are more collaborative and participatory.
Thanks to Steven Bell at Temple University for the original guide.