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 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 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 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