Open access (OA) refers to content freely available on the public internet. It often carries less restrictive copyright and licensing terms than traditionally published works, but it is copyrighted.
While OA is a newer form of scholarly publishing (which is commonly described as starting in 2002), 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.
Content described as OA can be in formats other than journal articles. For example, it might entail textbooks or other educational content in the form of Open Educational Resources (OER's).
pre-print: the version of a publication before it's been edited by peer-review
post-print: the version of a publication after it's been edited by peer-review but before it's been type set into the final version of record
version of record: the publication which is formally published, such as on a journal website
Green OA publishing refers to the self-archiving of published or pre-publication works for free public use, sometimes after an embargo period required by the publisher has ended. Authors provide access to preprints or post-prints (with publisher permission) in an institutional or disciplinary archive such as WM Publish and arXiv.org.
Gold OA publishing refers to works published in an open access journal and accessed immediately upon publication via the journal or publisher's website. Publishing costs ("article processing charges") if charged are often borne by the institution or by a subsidy. Examples of Gold OA include PLOS (Public Library of Science) and BioMed Central.
Bronze OA publishing refers to works available on websites hosted by their publisher either immediately or following an embargo but not formally licensed for reuse.
Platinum OA is a term sometimes used to refer to institutional subsidy or other external funding as opposed to author fees for publishing open access
Scholarly research indicates that not only are publishers offering more open access content from their own platforms, but they are increasingly receptive to publishing content which originally was posted as open access elsewhere. For example, they are often allowing for more posting of pre-prints (content before editorial review) as well as post-prints (content which has been edited but not formatted into a reprint). Generally, this is because scholarship goes under extensive editing between initial publication as a thesis or dissertation and acceptance into a peer-review publication. The articles below speak to this effect.