Open Access literature is defined as being freely available online in a digital format. Traditional publishing models where a subscription to a journal is required to view research creates a barrier between information and the public. Open access (OA) allows research to be more easily shared between researchers and with the public.
Types of Open Access Publishing
Green - publishing in a non-OA journal but maintaining the rights to the works and archiving in an OA repository. This allows authors to meet the public access requirements that many research funding agencies have.
Gold - publishing directly in an OA journal, allowing more immediate access to articles than green publishing.
Hybrid - Some subscription/closed-access journals have an "open option" that makes an article free available to everyone.
There are two more distictions to keep in mind between OA resources: libre and gratis. These terms refer to the amount of licensing restrictions a work has:
Libre OA materials are free of most licensing and copyright restrictions on use of the work.
Gratis OA materials are free to use, but still have licensing and copyright restrictions.