Open Access and Open Education Resources by Mike Bloomberg is licensed under CC BY 4.0
OER, or Open Educational Resources, are "teaching and learning materials that you may freely use and reuse at no cost, and without needing to ask permission" (OER Commons). Most often, these will take the form of materials that have been openly licensed, usually with a Creative Commons license, which means that their creators have allowed reuse of their works under some conditions.
Specifically, openly licensed works allow for these five freedoms, commonly called the 'five Rs':
These five freedoms offer many advantages in the classroom.
There are so many reasons to adopt OER in your classroom!
As you explore OER, you may wish to apply the principles of open education to more than just your textbook through open pedagogy, which is "the practice of engaging with students as creators of information rather than simply consumers of it" (thanks to the University of Texas at Arlington for that definition!). Using the Five Rs, students can create and adapt information even as they learn it.
These databases contain material from many different OER producers.
Don't forget that Open Access journal articles can also be used as Open Education Resources - provided the license of the article allows the "5Rs".