There are two primary classifications of open access: Gold or Green
Further definitions below:
The goal of Open Access is to digitally publish scholarly work making it easily accessible and free to use responsibly by the public. Authors retain control of the integrity of their work and the right to be properly cited and acknowledged. This way of publishing scholarly works looks to supplement or in some cases replace the current pay-for-access journals that limit access through price and permission barriers. Open Access hopes to restore the original intent of scholarly publication by removing these barriers and getting scholarly work seen and used by the most people possible.
Why open access?