The term ‘Open Educational Resources’ (OER) describes any educational resources (including curriculum maps, course materials, textbooks, videos, multimedia applications, podcasts, or any other materials designed for teaching and learning) which have been made available for use under open licenses – which means that anyone can use, adapt and redistribute them. Those materials may be available online, in print, on DVD or any other carrier.
OER are not the only educational materials to be found on the Web. Unfortunately, most resources on the Internet are closed resources, even if they are available for free. Materials that are under full copyright, or which are not accompanied by a specific license allowing anyone to copy, adapt and share them, are not Open Educational Resources. You can use these materials only within the fair use provisions or copyright exceptions in your country.
OER is not the same as Open Access (or OA).
More information about the definitions of Open Educational Resources can be found on the Creative Commons Wiki and in David Wiley's post on freedoms in OER.
This answer is adapted from Open Educational Resources Mythbusting by Karolina Grodecka and Kamil Silwowski, available under Creative Commons Attribution 4.0 International license (CC-BY).