Open Access means that the resource is freely available for viewing. There is a continuum of open access levels ranging from purcase only access to completely free and open no barrier publishing. Many publications that are subscription only may have an open-access option for some works. Open access is not related to the quality of the material or the peer-review status of the publication.
Green OA - refers to a level of open access where the author may self-archive published or pre-published works for free public use including in an institutional or disciplinary archive (dependent on publisher contracts).
Gold OA - refers to a level of open access where the work is published in an entirely open access journal.
Hybrid OA - refers to a journal that uses multiple economic models for publication including individual author fees to make their work open-access.
Some open access materials can be found indexed in library owned databases. The following resources are all open-access.