Gold Open Access journals that are available to their readers free-of-charge from the moment of publication, without embargo or restriction. Authors may or may not pay an article-processing charge to have their articles published in Gold OA journals.
In Green OA, Open Access takes place at the repository level, with journal policy allowing authors to archive pre-print and post-print versions of their articles. (Source: Sherpa) Contrast with Gold OA where journals provide Open Access to articles at the moment of publication.
Libre OA is Open Access that is free of price barriers as well as (at least some) permission barriers. You may reuse, republish, and remix the original work without permission from the creator or copyright-owner.
Gratis OA is Open Access that is free of price barriers but not permission barriers, so it allows for free online access to works but not reuse, republication, or remixing of the original work.
Older names for Libre OA and Gratis OA. Strong OA is free of price barriers and at least some permission barriers. Weak OA is free of only price barriers.
With Hybrid OA, some traditional journals (i.e., generally those that are published by for-profit scholarly presses) offer an option for authors to make their individual articles freely accessible to anyone worldwide, for an additional fee. Other articles in the journal remain accessible only through subscription. This is known as double-dipping, making an institution, author, or researcher pay twice to access the work.