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.
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. Students may see optional textbooks on their booklist when low-cost print copies are sold in addition to the free online access.