A Very Brief Introduction to Open Access by Peter Suber
http://www.earlham.edu/~peters/fos/brief.htm
Open-access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions. What makes it possible is the internet and the consent of the author or copyright-holder. OA is entirely compatible with peer review, and all the major OA initiatives for scientific and scholarly literature insist on its importance. Just as authors of journal articles donate their labor, so do most journal editors and referees participating in peer review. OA literature is not free to produce, even if it is less expensive to produce than conventionally published literature. The question is not whether scholarly literature can be made costless, but whether there are better ways to pay the bills than by charging readers and creating access barriers. Business models for paying the bills depend on how OA is delivered.
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Open Access Publishing (Gold): The characteristics of open access publishing are akin to traditional publishing, by which an author submits a work to be peer reviewed and is published at no cost for others to read. Costs to fund the publication process may be assessed to the author through article processing charges, although many publishers charge no fee.
Open Access Self-Archiving (Green): Green open access involves the self-archiving of research published through traditional journals. A variety of platforms are available for researchers to self-archive their work, including disciplinary repositories (e.g. ArXiv or PubMed Central) and institutional repositories (e.g. UJIR) hosted by universities and organizations.
Hybrid Open Access Journals: Hybrid Open Access Journals include subscription journals which offer an option to publish open access.
Article Processing Charges (APCs) may be defined as the charges levied by publishers from authors for publishing journal articles. The levy is generally set by commercial publishers at between a few hundred and a few thousand euros or US dollars per article.
https://www.library.cmu.edu/content/what-article-processing-charge-apc
