OERs are educational materials that include permission for anyone to use, modify and share at no cost. These are either in the public domain or have licenses that provide permission to retain, remix, revise, reuse and redistribute with proper citation. Resources with Creative Commons licenses meet these criteria. Check out some examples of OER remixed textbooks. Browse the list of WTCS OER textbooks or the WISELearn statewide repository.
AERs are free or low-cost materials that do not have open licenses for editing or sharing (but they are still awesome for students). Examples include free (non-open) web resources, Library-supplied resources, or Bookstore options under $40.