According to the Budapest Open Access Initiative adopted in 2002, "Open Access" (OA) refers to scholarly literature that is :
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.
Basically, OA is an attempt to remove barriers to accessing scholarly literature, so that this research is not only available to those who can afford it. Most OA literature is in the form of articles, though there have been some attempts to create OA books.