Gold: Published in an open-access journal (as defined by DOAJ).
Green: Toll-access on the publisher page, but there is a free copy in an OA repository.
Hybrid: Free under an open license in an toll-access journal.
Bronze: Free to read on the publisher page, but without a license.
Open Education Resources (OER) as defined by the Creative Commons are "free and openly licensed educational materials that can be used for teaching, learning, research, and other purposes". Essentially, these are teaching materials that you can use in your classes for free.
OERs come in many forms, from simple handouts or video clips to whole complete textbooks or courses. There are many places to find OERs online and many OERs are even peer-reviewed. Of course you can always create your own to share with others too.
"Open-access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions." (Peter Suber's Overview). Peer review is still an important part of open access. The goal of the open access is to remove barriers to access because of price or permissions, it is not about quality.