Open access refers to freely available, digital, online information. Open access scholarly literature is free of charge and often carries less restrictive copyright and licensing barriers than traditionally published works, for both the users and the authors.
While OA is a newer form of scholarly publishing, many OA journals comply with well-established peer-review processes and maintain high publishing standards.
Harvard University's Peter Suber is one of the leaders in the open access movement. His book, Open Access (published by MIT Press in 2012), provides an excellent overview of open access publishing and its benefits. This is an open access book and can be downloaded or read online for free.
Suber has also published a short overview.