Open educational resources (OER) are materials used for a teaching and learning purpose that can be freely downloaded, edited, and shared by others.
Just because a resource is free to view online does not make it an open educational resource. One of the most important aspects of OER is that they carry legal permission for open use, which is typically defined in terms of the 5R activities (Retain, Revise, Remix, Reuse, and Redistribute).
There are two ways that a resource may carry these permissions: 1) the work is part of the Public Domain or 2) the creator has assigned an open license. The most widely used type of open licenses are the Creative Commons (CC) licenses. (While there are a few alternatives to Creative Commons licenses, they are more commonly used with open software.)
Some examples of "free" learning materials that are not OER: