*Update* Please see Stevan’s comment below which clarifies some points I didn’t get right!
Open Access (OA) publishing means that the research paper or other information may be accessed by a reader without payment, unlike much scholarly research which is published behind a paywall. Paywalls are often invisible to university members, as they can click though without logging in if they are on-campus (much paywall access is mediated by IP addresses), but the such subscriptions are still paid by their institutions and they can be pretty expensive!
The main argument for OA is that much research is paid for by taxpayers through government grants, so it is argued that the end product of publicly-funded research should be freely available for citizens/taxpayers to read. Furthermore, academics in many fields often wish to continue to keep up-to-date in their subject after they have retired. They may still contribute to professional organisations and contribute papers and letters, but if they are no longer current members of a university, they lose institutional access entitlement to scholarly resources and thus their experience and expertise is lost to that field of study after they cease paid work for that institution.
Jackie Wickham (Nottingham University) spoke about research repositories and two routes to open access (OA) publishing: the more established gold model and the emerging green model.
The gold route is also known as the ‘author pays’ model, and it means that the publication of an article in an OA journal is usually paid for by the author’s institution or included in their research grant. Increasingly, UK Universities have established publication funds e.g. Nottingham and Birmingham. There are two further options for the gold route: OA publishers (PLOS, BioMedCentral, Hindari) or traditional publishers with OA option (Nature, Elsevier, Springer).
The green route involves self-archiving the article or conference paper in a repository of published research. This may be done by subject (e.g. PubMedCentral, arXiv, Repec) or by institution (also providing a way for universities to showcase their research and to preserve it). There is no charge for depositing the article or paper; the costs of running the archive are met by the institution.
The green route is usually used in addition to publishing in a journal, which may be OA, subscription or a hybrid of the two.
A problem with the green route is that the commercial journal publisher may impose an embargo on the publication of the article anywhere else but in their journal. As I understand it, the solution to this is that a very similar but not identical version of the article is presented to the commercial publisher and to the repository.