Open Educational Resources are teaching, learning, and research resources...that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation and redistribution by others with no or limited restrictions.
For more definitions of OER, see the Creative Commons Wiki
Source: The Review Project
There are many reasons instructors might want to use Open Educational Resources:
There have been multiple studies on faculty implementations, misunderstandings, acceptance of, and evaluation of OER. The Review Project has curated a number of empirical studies published in scholarly journals on the topic. Their general conclusions:
Across multiple studies in various settings, students consistently reported that they faced financial difficulties and that OER provided a financial benefit to them. A general finding seems to be that roughly half of teachers and students find OER to be comparable to traditional resources, a sizeable minority believe they are superior, and a smaller minority find them inferior.
In total, more than 25,000 students have utilized OER materials across the studies that attempted to measure results pertaining to student efficacy. These students results were compared with approximately 100,000 students using traditional textbooks. While causality was not claimed by any researcher, the use of OER was sometimes correlated with higher test scores, lower failure, or withdrawal rates. In only one efficacy study did more students do worse than did better, and even in that study the majority of students achieved the same results as their peers using traditional textbooks.