Unlike traditional publishing, Open Access journals are supported by copyright holders who freely and generously allow wide distribution to all users. Reproduction in any medium is allowed and encouraged provided the original author and source are credited. Thousands of Open Access journals are available full text in
PubMedCentral, http://www.ncbi.nlm.nih.gov/pmc/journals/ and
Open Access is usually something the author decides to grant upon publication, whereas Public Access is a mandate about NIH funded research articles.
Beware of predatory Open Access journals; for more information visit: http://libraryguides.mayo.edu/predatory-publishers