Open access means free public online access to research literature - continuing the existing generous practices of scholars who volunteer their writing, review and editing services. There are many paths to open access, and although none are cost-free, many significantly reduce distribution costs while maintaining high levels of rigor and quality and simultaneously increasing research access and visibility.
Open access to research articles can be achieved through publishing only in journals that are fully open ("gold" open access), through subsidizing open access to an individual article in a publication that is otherwise toll-access ("hybrid" open access), or through scholars sharing their articles in institutional or subject-based repositories ("green" open access).
Some organizations that fund research require that published results be made openly available - the National Institutes of Health is one example, as is the U.K.'s Wellcome Trust. Many research institutions require or ask that their employees make published articles openly available. Such requirements are often referred to as "open access policies." As of January 1, 2015, the University of Minnesota has an open access policy.
While open access advocacy (and funder and institutional policies) tends to focus on journal literature, some academics also explore options for making their monograph publications freely publicly available.
MIT's Open Access FAQ
Springer's Open Access Glossary
The Open Access Directory wiki