OER stands for Open Educational Resources, and is a term that refers to any educational resources that are typically:
Open Educational Resources are teaching, learning, or research materials offered freely and openly for anyone to use. The term OER generally refers to digital resources and may be in the public domain, or offered under an open license that allows for reuse, revision, remixing, and/or redistributing.
What is NOT OER
A specific type of OER designed to be a free or low cost substitute for a traditional textbook.
(Source: Kirkwood CC, Anne Arundel Community College)
Here are some examples of different types of open textbooks.