"Open Educational Resources (OERs) are any type of educational materials that are in the public domain or introduced with an open license. The nature of these open materials means that anyone can legally and freely copy, use, adapt and re-share them. OERs range from textbooks to curricula, syllabi, lecture notes, assignments, tests, projects, audio, video and animation". (cited from UNESCO)
OERs are learning and teaching materials which are online open access. They are published under open licenses (i.e Creative Commons) that specify how materials can be used, reused, adapted, shared and modified according to specific needs.
For more detailed information about Open Educational Resources including management, learning and teaching, legal and technical considerations, see the JISC guide on OERs.