Open access (OA) means unrestricted online access to peer-reviewed scholarly research. Open access is primarily intended for scholarly journal articles, but is also provided for a growing number of theses, book chapters, and scholarly monographs.
Open access comes in two degrees: gratis open access, which is free online access, and libre open access, which is free online access plus some additional usage rights. These additional usage rights are often granted through the use of various specific Creative Commons licenses. Only libre open access is fully compliant with definitions of open access such as the Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities.
The two ways authors can provide open access are (1) by self-archiving their journal articles in an open access repository, also known as 'green' open access, or (2) by publishing in an open access journal, known as 'gold' open access. With green open access authors publish in any journal and then self-archive a version of the article for gratis public use in their institutional repository, in a central repository (such as PubMed Central), or on some other open access website. With gold open access, authors publish in open access journals, which provide immediate open access to all of their articles, usually on the publisher's website. (from the Wikipedia page on Open Access. 2/13/2015)