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.
There's a lot of misinformation out there about Open Access. People believe OA publications aren't peer-reviewed, have low impact factors, and require author fees. The truth is that while there are some poor quality OA publications, there are several high quality, high impact, well respected OA publishers as well. Some of them require author fees, but most do not. It's important to remember that both of those statements could also be made about subscription publications. Here are a few links to some OA myth de-bunking articles:
These are some of the most highly regarded providers of OA literature: