Open access (OA) 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, legitimate OA journals and books comply with well-established peer-review processes and maintain high publishing standards.
Green OA: the self-archiving of published or pre-publication works for free public use. Authors provide access to preprints or post-prints (with publisher permission) in an institutional or disciplinary archive such as eCommons@Cornell and arXiv.org.
Hybrid OA: a journal that publishes many or most articles using a traditional subscription model, but allows authors to pay an extra fee to make their article OA.
Delayed OA: an article that becomes OA after a set time period, typically 6 or 12 months.