What is the difference between open access literature and digital, online and free of charge literature?

Digital, online and free for users literature doesn’t have the price barriers for the users, but still has permission barriers (e.g. registration, copyright and licensing restrictions, no reuse rights). E.g. you might have free access to research literature via HINARI, AGORA, OARE and other international initiatives because somebody paid on your behalf, or the publisher was generous to provide free access to you, or this was a result of negotiations. If you are asked to register, provide IP address, or sign a license, this is not open access.

By 'open access' to literature, we mean its permanent 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 definition from the Budapest Open Access Initiative). 

More information: see Briefing Paper What is Open Access? written by Alma Swan for OASIS [PDF]