"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. Open educational resources include full courses, course materials, modules, textbooks, streaming videos, tests, software, and any other tools, materials, or techniques used to support access to knowledge." 
The Open Education movement is built around the 5Rs of Open 
OERs are educational materials that are specifically designed by their creator/s to be openly available, and are often licensed to be re-used, re-mixed, and re-distributed. Open is not just about low cost (though that is an important benefit of using OER) but about the ability to take what others have created, customize it for your specific educational needs, and then share your creation with others.
OERs can come in a variety of forms:
The open resource movement has been around for a while, starting with static learning objects (about 2000), and transitioning to OERs that allowed for revision and reuse. It is the ever increasing cost of textbooks and materials for students that is now pushing the OER movement forward. Textbooks and learning materials cost students approximately $1,100 per year. The College Board statistics say 800% since 1980. According to the Chronicle of Higher Education, 7 in 10 students didn't purchase a textbook because it was too expensive. Students often have to wait several weeks into a semester for financial aid in order to purchase textbooks, missing valuable lesson time. Through OERs the cost of student materials can be drastically reduced. OERs also give instructors the ability to customize the materials, creating the "perfect" textbook instead of being bound to traditional print resources.