OER are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others. 

Source: The William and Flora Hewlett Foundation

For more definitions of OER, see the Creative Commons Wiki.

1. OER can either be in the public domain, or under a more lax intellectual property license.
2. OER can be revised, remixed, added upon, translated, and then shared again to meet different needs.
3. OER can take many forms, such as: syllabi, lesson plans, videos, software, tests, teaching techniques, group activities, writing prompts, textbooks, learning modules, experiments, simulations, and course designs. There are no platform restraints.

Source: The Review Project

There are many reasons instructors might want to use OER: 

Free and Legal to Use, Improve and Share

• Save time and energy by adapting or revising resources that have already been created
• Tailor educational resources to the specific content for your course
• Expand opportunities for interdisciplinary teaching and learning by allowing you to integrate and revise multiple educational resources
• Redefine "traditional" learning by incorporating multi-media or scenario-based education
• Go beyond the confines of "teaching to the book"

Network and Collaborate with Peers 

• Access educational resources that have already been "peer reviewed" by other experts in your field
• Review or annotation features and texts so other instructors have more in-depth knowledge of the resource and its quality quickly
• Make learning and teaching a team project using collaborative platforms

Lower Educational Cost and Improve Access to Information

• Reduce the cost of course materials, particularly textbooks so that all students have access and aren't as financially burdened
• Find and access information instantly on virtually any topic, on various devices.
• Give learners the option of looking at course content openly before enrolling.
• Reduce the load students bear, possibly increasing graduation and retention rates

Hoping to learn more? 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 conclusion is: 

Once adopted, OER provide the permissions necessary for faculty to engage in a wide range of pedagogical innovations. In each of the studies reported above, OER were used in manner very similar to the traditional textbooks they replaced. We look forward to reviewing empirical articles describing the learning impacts of open pedagogies.