Digital repositories function as "storehouses" of publications organized around an institution or discipline.
For example, the Iowa State University Digital Repository hosts scholarly and creative works, research, publications, and reports contributed by faculty, students, staff, and administrative units of Iowa State University while arXiv.org hosts papers (mainly) in the physical sciences. Content in repositories often includes peer-reviewed content (publisher's version or post-prints) as well as pre-prints, the version of an article before it under went peer review.
The publication and peer review process can take months if not years. Sometimes research results will prompt important changes before publication occurs. Depositing a pre-print into a repository is one way to share important results without waiting for publication.
Green OA compliments Gold OA by providing authors a way to share their work with others, even if they publish in a journal that is not OA.*
Repositories usually have stable sources of funding so works deposited into them remain accessible even if the authors are not. They are also often affiliated with well-known institutions such as universities or government agencies which makes them easy to find.
*There are still publishers who do not support any type of OA, Green or Gold.
As publishers are constantly revising their terms, the best way to find out if they support repositories and green OA is to read your publishing contract. If you sign away your copyright when you submit to a journal, you may be unable to self-archive. Understanding the vocabulary these publishers use is important.
The (draft) paper first submitted to publishers - it has not been through peer review or accepted for publication.
This paper has been through peer review and accepted for publication but has not been formatted by the publisher.
The final, published version of a paper with the publisher's typesetting, copy-edits, comments, supplemental data, hyperlinks, etc.
Some subscription journals have chosen to make all of their content open access after a set amount of time has passed. This time period is usually 1 - 5 years but can be as short as 2 months and as long as 10 years. Some embargoes only lift off of Preprints or Postprints, allowing authors to use Green OA, but some journals also lift the embargoes off of the Publisher's Version. Use SHERPA/RoMEO to see publisher's copyright policies about self-archiving.
The Iowa State University Digital Repository (DR) is Iowa State’s open access, full-text institutional repository. What this means is that the DR is a powerful tool to collect, manage, preserve and provide free, worldwide access to research and scholarship of Iowa State’s faculty, staff and students.
Materials available in the DR can be used by anyone with an internet connection, anywhere in the world. This frees your scholarship and provides your work with an international platform. Materials deposited in the Digital Repository are given a stable, permanent URL. This means that when you link to your works in the repository, or when your colleagues cite your research, users will always be able to find it.