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 (Peter Suber). OA is achieved through two primary channels: archiving (Green OA) or publishing (Gold OA). OA is completely compatible with peer review.
OA 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.
Gold OA - publishing open access
Green OA - open archiving, possibly embargoed or not version of record
Hybrid - article-level OA in traditional subscription access journals, usually for a fee
Preprint - version of a scholarly work submitted for peer review including only the original work of the author(s)
Post-print - also known as the author's final version, this version incorporates all changes from peer review, but it has not yet been copyedited and formatted for publication
Creative Commons - a nonprofit organization that creates licenses under which works are distributed with reuse permissions granted upfront
Addendum - attached to publication or copyright transfer agreements requesting additional author rights beyond those already granted by the publisher
Adapted from Molly Keener's Open Access Research Guide from Wake Forest University's Z. Smith Reynolds Library.