UNESCO, an early proponent of Open Educational Resources (OERs), defines them as:
"...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."
Basically, OERs are freely available resources meant for use in the classroom. Typically OERs are licensed under a Creative Commons license which allows free use and re-use with appropriate attribution.