The Open access (OA) initiative seeks to make research articles, research documents, scholarly-reviewed works, and other publications freely available on the Internet at no cost to the reader or for a relatively low fee if the reader prefers a print format. OA covers basically all areas of knowledge. The Open Access publishing model increases visibility and reuse of academic research results.
There are two mayor Open Access Publishing Models
1. Gold Open Access
Authors publish their works in Open Access journals. Some of these journals require no publication fees, but others, such as the gold open access, do. It involves publishing articles or books via the OA route on a publisher's platform which usually involves paying production costs. The costs are usually covered by the authors, their institutions, or the research organizations that funded their research. Contents published via the gold model are usually accessible immediately upon publication.
2. Green Open Access.
This is self-archiving whereby authors make the full text of their research work freely available in a trusted repository that is publicly accessible and managed by a research organization. The Digital Commons and the Adventist Digital Library are examples of Green Open Access. An author who has published via a commercial publisher could make the content of that publication available through OA when the self-archiving embargo period for the publication has elapsed.
Generally the terms of access of OA publications depend on the type of license under which the materials have been published. But they are usually generously licensed. A directory of fully open access journals is found in DOAJ website while that of open access books is found in DOAB.