Traditional, subscription-based publishing relies on university libraries to pay for access to journals. Only enrolled students and faculty can access the content of these journals. The cost of publishing is covered by the subscriptions.
An Article Processing Charge (or APC) is the fee to publish in some open access journals. (Not all open access journals or publishers charge a fee to publish an article.)
How much are APCs? An example
The University Libraries have an agreement with publishers to waive APCs in certain journals they publish. Currently, the three publishers with such agreements are Cambridge University Press, Elsevier, and Wiley.
Open Access (OA) is a model of publishing that allows unrestricted online access to scholarly journal articles.
Gold OA is where an entire journal is open access. Rather than the traditional subscription-based publishing model, the journal funds its operations using Article Processing Charges paid for by the authors.
Hybrid OA is where a journal that uses the traditional subscription-based publishing model allows authors the option to publish their article OA. The OA article will be marked open access but the rest of the journal articles will be behind a paywall. Publishing is paid for by Article Processing Charges, and the individual articles are free to the public. The author may or may not retain the copyright under Hybrid OA.
Green OA The publisher, who owns the copyright, may allow the author the right to self-archive on a personal website, an institutional repository, or a third-party repository, like ResearchGate or Academia.edu.