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 OA is considered a new form of scholarly publishing, many OA journals comply with well-established peer-review processes and maintain high publishing standards. Please see Peter Suber's thorough overview of Open Access on the following website: http://legacy.earlham.edu/~peters/fos/overview.htm.
Green OA publishing refers to the self-archiving of published or pre-published works for free public use. Authors will provide access to preprints or post-prints (pending publisher permission) in an institutional or discipline archive. Examples of green OA include eCommons@Cornell and arXiv.org.
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 PLOS (Public Library of Science) and BioMed Central.
Image: Opensource.com, http://tinyurl.com/l7y66vo