Open access 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 OA is a newer form of scholarly publishing, many OA journals comply with well-established peer-review processes and maintain high publishing standards. For more information, see Peter Suber's overview of Open Access: http://legacy.earlham.edu/~peters/fos/overview.htm.
Green OA - Refers to self-archiving of the pre-print or post-print article manuscript in repositories.
Gold OA - Refers to articles in fully accessible OA journals. Gold open access journals are supported by author-side article processing charges (APCs). Gold open access journals are not subscription based and charge an APC as a means of covering the costs of publishing. The funding model changes from “reader pays” to “author pays.”
Hybrid OA - Refers to subscription journals that offer the option for authors to make individual articles open access (freely available to everyone) by paying an APC fee.
Harvard University's Peter Suber is one of the leaders in the open access movement. His book, Open Access (published by MIT Press in 2012), provides an excellent overview of open access publishing and its benefits. This is an open access book and can be downloaded or read online for free.