Open access (OA) refers to freely available, digital, online information. Open access scholarly literature is free of charge and often carries less restrictive copyright and licensing barriers than traditionally published works, for both the users and the authors. 

While some OA publishers require authors to pay page charges to help defray the cost of publishing, OA is not vanity publishing or self-publishing since legitimate OA publishers still submit authors to the same rigors of peer review as traditional academic publishers.  For more information, see:

Green OA publishing refers to the self-archiving of published or pre-publication works for free public use. Authors provide access to preprints or post-prints (with publisher permission) in an institutional or disciplinary archive such as:

Gold OA publishing refers to works published in an open access journal and accessed via the journal or publisher's website. Examples of Gold OA include:

• Hybrid OA offer authors the option of making their articles open access, for a fee. Journals that offer hybrid OA are still fundamentally subscription journals with an open access option for individual articles. They are not true open access journals, despite publishers' use of the term "gold open access" to describe this arrangement..
• Diamond OA publishing describes journals that are completely free to publish and to read. The cost of maintaining and publishing the journal is usually borne by the organization that sponsors the journal. Diamond OA status has no impact on the journal's peer review process. By making articles completely free to both publish and to read, Diamond OA best approaches the goals of democratizing and widely distributing academic scholarship.
• Bronze OA publishing describes articles that are free to read on a publisher's homepage, but without clarity on the specific licenses covering an article. Bronze OA articles may be free to read due to a temporary publisher marketing campaign, for example.

   Image: Opensource.com, http://tinyurl.com/l7y66vo

• Gratis OA is information that is available free of charge, while some copyright and licensing restrictions may still apply.

• Libre OA is information that is free of charge and free of most copyright and licensing restriction.

• While 'free' implies that the information does not cost anything to access, remember that OA publishing still often involves a cost to the author to publish the work.

Copied, with changes and permission, from the "Open Access Publishing" guide, created by the Gail Steinhart and Jeremy Cusker at Cornell University.