OERs consist of any type of educational resource that is free for teachers and students to reuse and share. They include things like:
lectures (audio or video) and lecture notes
lesson plans
readings lists
tests and quizes
videos
games and activities
course assignments
Four R's of OER
The four R's are often used to describe OERs, because they can be:
Reused - used verbatim, exactly the way you found it
Revised - updated, or translated to other languages
Remixed - combined with existing things to produce something new
Redistributed - shared with others
Why are OER's unique?
OER authors retain few, if any, ownership rights. This means you can do anything you want with it. Materials often have a Creative Commons or GNU licenses (for software) that state specifically how items can be used.