The National Institutes of Health has issued a revised Public Access Policy to ensure compliance with Public Law 110-161. Under the revised policy, any peer-reviewed journal articles arising in whole or part from direct costs paid by the NIH or NIH staff must be submitted in electronic format to the National Library of Medicine's digital archive PubMed Central (PMC) and made available within twelve months of publication.
Open Access is "a movement to make research works such as articles, data, software, etc., available freely, immediately, and permanently online to the public without financial and technical barriers." (NNLM Data Thesaurus)
More general information on what Open Access is: