According to the William and Flora Hewlett Foundation, Open Educational Resources are "high-quality teaching, learning, and research materials that are free for people everywhere to use and repurpose."
According to UNESCO, 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. OER range from textbooks to curricula, syllabi, lecture notes, assignments, tests, projects, audio, video, and animation.
According to SPARC (the Scholarly Publishing and Academic Resources Coalition), the usage of OER is critical to help address the rising costs of textbooks, which is causing many students to forgo the purchase of highly priced textbooks for their courses. SPARC believes that OER can help leverage the digital environment and open up education even further (SPARC Factsheet on OER).
For more information on the importance of OER, see the following documents: