Open educational resources depend on licensing that allows the copyright owner to freely distribute their materials under certain terms. The CC license puts certain restrictions on materials; there are different permissions associated with each license. Before using OER materials for your class, examine the CC license for attribution details and rights for re-mixing or adaptation.
SPARC (the Scholarly Publishing and Academic Resources Coalition) works to enable the open sharing of research outputs and educational materials in order to democratize access to knowledge. SPARC members are primarily academic and research libraries promoting open access to materials.
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.
Read The Review Project, a summary of empirical research studies on the impact of OER adoption from the Open Educational Group.