Open Educational Resources (OER) are resources that are freely accessible and "have been released under an intellectual property license that permits sharing, accessing, repurposing -- including for commercial purposes -- and collaborating with others" (OER Commons).
OER titles aren't just free and under an open license, they are also scholarly resources for teaching, learning, and research. OERs can range from textbooks to journals, films and videos, to even online courses. Despite their free and open status, they are written by scholars and are mostly peer-reviewed.