Open Access is the free, immediate, online availability of learning materials, research, datasets, and creative work. When we talk about Open Access, we typically mean journal articles and monographs that are published under an open license and are available for anyone to access and reuse, without paywalls, logins, or other barriers to access.
Open Access can be achieved through two primary channels: archiving (Green OA) or publishing (Gold OA).
Green Open Access archiving is the intentional act by content producers to make copies of their works available in a freely-accessible digital archive or repository. Repositories may be affiliated with funding agencies (PubMed Central) or institutions (Research Discovery), or may be subject-specific (arXiv). Although archiving of the final published version is the ideal, most archived works are either preprints or post-prints. Pre-prints are versions of works as submitted for review, and includes only the original work of the author(s). Post-prints, or the author's final version, are versions that incorporate all changes from the peer review process, but have not yet been copyedited and formatted for publication (i.e., no page, volume or issue numbers, etc.).
Gold Open Access publishing shifts the costs of content creation and distribution away from subscribers. What this means is that OA published information is freely available to anyone worldwide with an internet connection—no subscription necessary. Instead of relying on subscriptions to subsidize publishing costs, those costs are covered by sponsorships or author-side fees, which may be covered by institutions or grant-funding organizations. Fewer than half of all OA journals charge author-side fees, and many of those that do will waive such fees for authors with economic hardship.
By removing barriers to access, Open Access publishing makes your work more visible. This increased visibility leads to a demonstrated increase in downloads and citations of OA articles compared to those published by subscription journals. People all over the world are able to read your research, freely share it with others, build on what you have written, and create networks of academics doing similar work.
Benefits of Open Access (Sources: SPARC, righttoresearch, Kingsley & Brown 2012)