The Budapest Open Access Initiative defined Open Access (OA) as the “availability on the public internet, permitting any users to read, download, copy, distribute, print, search, or link to the full texts of articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself. The only constraint on reproduction and distribution and the only role for copyright in this domain should be to give authors control over the integrity of their work and the right to be properly acknowledged and cited."
Open Access (OA) refers to freely available online information that can be fully used in the digital environment. OA resources are free of charge; authors keep their copyright; there is no embargo to the information; and an open license (such as Creative Commons) is usually applied.
You may have heard about Open Educational Resources, or OER.
OA and OER are closely related. Both use open licenses (such as Creative Commons) that make OA and OER freely available online. Likewise, OA and OER are both used in educational settings:
OA supports faculty and students by providing them with free, online access to up-to-date research. OA resources are available after graduation for lifelong learning;
generally deals with academic publishing. It allows for authors to publish in peer-reviewed journals or self-archive in repositories.
Green OA = the work (usually post or pre prints) is made freely available in a repository, such as the CSU ePress.
Gold OA = the final version of the work is made freely available by the publisher, typically by publishing in an Open Access journal and making the article available under an open license (such as Creative Commons).