Open Access means to make research available online with no cost to the reader. Most funders require research outputs to be made available for free to maximise the benefits of the research. Open access is also a requirement for submissions to the REF (Research Excellence Framework).
There are several ways to make something open access:
Green Open Access
This is also known as self-archiving, and involves the author archiving a pre-print or Author's Accepted Manuscript (AAM) in an institutional repository or subject specific archive. There are no costs involved with this, but there may be an embargo set by the journal. To keep the monopoly on a published article, journals often add an embargo, which means that the AAM cannot be shared online until the embargo is over. The length of an embargo varies and depends on the journal's policy, but the most common length is between 3 to 12 months.
Gold Open Access
Gold Open Access is to publish with an Open Access journal. The article is freely available online for the reader on the time of publication. Many Open Access journals have an Article Processing Charge (APC), which is set to cover the cost of publishing as this is not covered by subscription fees. The APC varies depending on the subject area and quality of the journal, and the average APC is around £2,000. In open access publishing, the author keeps the copyright.
A hybrid journal is a subscription based journal that allows open access publishing. This means that an author can pay an APC to make their article open access. Hybrid journals have received a lot of criticism as they have been accused of 'double dipping' as they charge APCs as well as subscription fees, and rarely adjust their subscription fees according to the extra income from the APCs.
Diamond Open Access
This is almost identical to Gold Open Access, apart from the APC. In Diamond Open Access publishing there is no fee for publishing, and these journals are often funded by organisations, institutions or other initiatives.