OPEN ACCESS is:
The Budapest Open Access Initiative, a 2002 landmark document in the OA movement states:
There are many degrees and kinds of wider and easier access to this literature. By "open access" to this literature, we mean its free availability on the public internet, permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself. The only constraint on reproduction and distribution, and the only role for copyright in this domain, should be to give authors control over the integrity of their work and the right to be properly acknowledged and cited. 
Open access literature includes both peer-reviewed journal articles and author self-archived pre- and post-prints deposited in digital repositories.
Even though most journals receive articles and labor (in the form of peer review and other activities) free of charge from scholars, there are still significant costs associated with scholarly publishing. The switch from a paper to electronic model lowers these costs, but doesn't do away with them altogether. The considerable expenses of orchestrating the publication process, and ensuring that articles remain accessible through technology maintenance and upgrades, still need to be funded.
The open access model shifts this funding from the point of access (e.g. subscription fees) to the point of dissemination . Various methods have been proposed for achieving this shift, but the most common is charging a publication fee for each article, ranging from $500 to $1500 .
It is not, of course, expected that scholars would pay these fees out of their own pockets. Instead, the idea is for the cost of publication to be built into grant proposals, so that the institution funding the research bears the expense. The founders of Public Library of Science--the most successful and visible OA publisher to date--argue,
The institutions that sponsor research intend for the results to be made available to the scientific community and the public. If these research sponsors also paid the essential costs of publication--amounting, by most estimates, to less than 1% of the total spent on sponsored research--we would retain a robust and competitive publishing industry and gain the benefit of universal open access. 
Already, the Howard Hughes Medical Institute, one of the largest research funding bodies in the United States, has begun a policy of allocating $3000 to grantees to cover the cost of OA publishing .
OA journals are not the only way to achieve free access to scholarly literature. Even if you are publishing in a commercial journal, you may still be able make your work freely available online by depositing it in a digital repository. In fact, a strong majority of publishers allow this practice (you can check the policies of specific publishers using the SHERPA database).
While increased public access to research is a noble goal in and of itself, it also directly benefits scholars and serves their career goals by increasing the impact of their writing. In 2001, Nature published a letter from Steve Lawrence with the title "Free online availability substantially increases a paper's impact." In if, Lawrence outlined his findings: articles freely accessible on the public Internet are cited at a significantly higher rate than those only available via paid subscriptions . While these findings were compelling, the study was limited to the field of computer science, and was undertaken at a relatively early point in the OA movement. A more recent study, published in the September 2004 College & Research Libraries, looked at articles "in four disciplines at varying stages of adoption of open access-philosophy, political science, electrical and electronic engineering, and mathematics." It found that, in all areas, freely available articles have greater research impact, as measured by ISI Web of Science citations . So, even at this relatively early point in online publishing, there is already strong evidence that, if you make your research freely available, it will be more widely cited.
Excerpted from Peter Suber article published in The Guardian, 2013 (with edits and additions.)
1) The only way to provide open access to peer-reviewed journal articles is to publish in open access journals
Open access delivered by journals is called “gold” open access and open access delivered by repositories is called “green” open access. The myth asserts that all open access is gold , even for peer-reviewed articles. However, the belief that publishing open access means publishing only in open access journals is false.
2) All or most open access journals charge publication fees
About one-third of open-access journals charge publication fees (compared to three-fourths of conventional journals).
3) Most author-side fees are paid by the authors themselves
According to the comprehensive Study of Open Access Publishing (SOAP), when researchers publish in fee-based open access journals, the fees are paid by funders (59%) or by universities (24%). Only 12% of the time are they paid by authors out of pocket.
4) Publishing in a conventional journal closes the door on making the same work open access
Most conventional publishers give standing permission for author-initiated green open access. Many of the others will give permission on request. For authors unsure of a publisher’s position, check out the Sherpa RoMEO database of publisher policies, read the publishing contract, or ask an editor. It’s always worth asking, if only to register demand and show rising expectations.
5) Open access journals are intrinsically low in quality
As early as 2004, Thomson Scientific found that in every field of the sciences “there was at least one open access title that ranked at or near the top of its field” in citation impact.
6) Open access mandates infringe academic freedom
This is true for gold open access but not for green. But if you believe that all open access is gold, then this myth follows as a lemma. Because only about one-third of peer-reviewed journals are open access, requiring researchers to submit new work to open access journals would severely limit their freedom to submit work to the journals of their choice. By contrast, green open access is compatible with publishing in non-open access journals, which means that green open access mandates can respect author freedom to publish where they please. That is why literally all university open access mandates are green, not gold. It’s also why the green/gold distinction is significant, not fussy, and why myths suppressing recognition of green open access are harmful, not merely false.
7) Open Access Journals are not peer-reviewed
Quality Open Access journals have peer-review, provide an option to request a waiver to OA publishing fees (if applicable), and are a member of the Open Access Scholarly Publishers Association or adhere to its Code of Conduct. However, predatory OA publishers do exist, and it is good to know what clues to look for to avoid predatory publishers.
8) Publishing in an Open Access venue means that I have retained all my copyrights
 Suber, Peter. "Open Access Overview."
 Budapest Open Access Initiative. 2002.
 Prosser, David C. "The Next Information Revolution - How Open Access Repositories and Journals will Transform Scholarly Communications." SPARC Europe, 2003.
 Guterman, Lila. "The Promise and Peril of Open Access." Chronicle of Higher Education, January 30, 2004 (Volume 50, Issue 21, Page A10).
 Brown, Patrick O., Michael B. Eisen, and Harold E. Varmus. "Why PLoS Became a Publisher." October 13, 2003. (Also published in PLoS Biology, Volume 1, Issue 1, October 2003.)
 Zandonella, Catherine. "The Economics of Open Access." The Scientist, August 22, 2003.
 Lawrence, Steve. "Free Online Availability Substantially Increases a Paper's Impact." Nature Web Debates.