"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". (UNESCO)
OERs are teaching materials which are openly available online. While they are open, they are not "free" of restrictions. They are covered by open licenses that specify how they 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 and take a look at Univerisity of Massachussets Darmouth OER LibGuide.