(Source: Peter Suber at: http://www.earlham.edu/~peters/fos/overview.htm)

Open-access (OA) literature is digital, online, free of charge (subscriptions, licensing fees, pay-per-view fees), and free of most copyright and licensing restrictions. In addition to removing access barriers, Open Access is immediate, rather than delayed, and should apply to full-text, not just to abstracts or summaries. OA is compatible with copyright, peer review, revenue (even profit), print, preservation, prestige, career-advancement, indexing, and other features and supportive services associated with conventional scholarly literature. The primary difference is that the bills are not paid by readers and hence do not function as access barriers.

There are two primary vehicles for delivering OA to research articles, OA journals and OA archives or repositories. Most activists refer to OA delivered by journals as gold OA, and to OA delivered by archives or repositories as green OA. The gold/green distinction is simply about venues, not user rights or degrees of openness.

OA serves the interests of many groups including:

Authors: OA gives them a worldwide audience larger than that of any subscription-based journal, no matter how prestigious or popular, and provably increases the visibility and impact of their work.

Readers: OA gives them barrier-free access to the literature they need for their research, not constrained by the budgets of the libraries where they may have access privileges. It increases their convenience, reach, and retrieval power.

Teachers and students: OA puts rich and poor on an equal footing for these key resources and eliminates the need for permissions to reproduce and distribute content.

Libraries: OA solves the pricing crisis for scholarly journals. It also solves what I've called the permission crisis. OA also serves library interests in other, indirect ways. Librarians want to help users find the information they need, regardless of the budget-enforced limits on the library's own collection. University librarians want to help faculty increase their audience and impact and thereby help the university raise its research profile.

Universities: OA increases the visibility of their faculty and institution, reduces their expenses for journals, and advances their mission to share knowledge.

Journals and publishers: OA makes their articles more visible, discoverable, retrievable, and useful. If a journal is OA, then it can use this superior visibility to attract submissions and advertising, not to mention readers and citations. If a subscription-based journal provides OA to some of its content (e.g. selected articles in each issue, all back issues after a certain period, etc.), then it can use its increased visibility to attract all the same benefits plus subscriptions. If a journal permits OA through postprint archiving, then it has an edge in attracting authors over journals that do not permit postprint archiving. Of course subscription-based journals and their publishers have countervailing interests as well and generally oppose OA. But it oversimplifies the situation to think that all their interests pull against OA.

Funding agencies: OA increases the return on their investment in research, making the results of the funded research more widely available, more discoverable, more retrievable, and more useful. OA serves public funding agencies in a second way as well, by providing public access to the results of publicly-funded research.

Governments: As funders of research, governments benefit from OA in all the ways that funding agencies do (see previous entry). OA also promotes democracy by sharing government information as rapidly and widely as possible.

Citizens: OA gives them access to peer-reviewed research (most of which is unavailable in public libraries) and gives them access to the research for which they have already paid through their taxes. It also helps them indirectly by helping the researchers, physicians, manufacturers, technologists, and others who make use of cutting-edge research for their benefit.