Open access (OA) refers to freely available, digital, online information. Open access scholarly literature is free of charge and often carries less restrictive copyright and licensing barriers than traditionally published works, for both the users and the authors.
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 overview of Open Access.
Attribution (BY)
You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
Non-commercial (NC)
The material cannot be used for commercial purposes.
Share Alike (SA)
If you remix, transform or build upon the material, you must distribute your contributions under the same license as the original.
No Derivative Works (ND)
If you remix, transform or rebuild upon the material, you may not distribute the modified material.