OER are "teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others."
(From The William and Flora Hewlett Foundation)
OER include a wide variety of course material types, including open textbooks, modules, syllabi, lectures, homework assignments, quizzes, lab activities, games, and simulations.. They are either (a) in the public domain or (b) licensed in a manner that provides everyone with free and perpetual permission to engage in the 5R activities: Retain, Reuse, Revise, Remix, Redistribute.
There are a few options for using OER: You may adopt materials as-is; adapt materials to better meet your needs; or create materials to share openly with other instructors.