At it’s most fundamental Open Access is when publications are freely available online to all at no cost, and with limited restrictions with regards reuse. The unrestricted distribution of research is especially important for authors (as their work gets seen by more people), readers (as they can access and build on the most recent work in the field) and funders (as the work they fund has broader impact by being able to reach a wider audience).
While OA is a newer form of scholarly publishing, many OA journals comply with well-established peer-review processes and maintain high publishing standards. For more information, see Peter Suber's Open Access Overview:
In general, there are two streams of Open Access Publishing, Gold and Green.