Open Access (OA) content, which includes open scholarly articles, open data, open research, open education, is digital, online, free of charge, and free of most copyright and licensing restrictions.
There are different ways of engaging with open access -
There are two primary vehicles for delivering OA to research articles, OA journals ("gold OA") and OA repositories("green OA"). The chief difference between them is that OA journals conduct peer review and OA repositories do not. Institutional Respositories can contain peer-reviewed journal articles that are published elsewhere. They can also contain pre-prints and post-prints of scholarly published articles.
There is also Gratis OA and Libre OA. Gratis OA removes no permission barriers and Libre OA removes one or more permission barriers. (Both of them remove price barriers.)