As the Open Access model becomes increasingly important for public knowledge dissemination, some agencies with vested interests have begun to complicate matters by introducing hybrid publishing models. Some of these are not fully in the interests of authors or readers.
PLOS has a great discussion about the issues at stake, and they refer to the OAS brochure, which is provided in many languages.
The second page of this OAS brochure is very short, clear and helpful. Recommended!