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.
Definition from UNESCO. The term Open Educational Resources (OER) was coined at a 2002 UNESCO Forum on the Impact of Open Courseware for Higher Education.
OER can take many forms. Here are some examples: