The term "digital repository" is used for a stable infrastructure where scholars may post and preserve research or the data that supports research.  Although these types of repositories may have flexible tools for limiting or expanding access to specific documents, they exist primarily to promote open access.  When you use Google Scholar, many of the articles you are able to download are the result of the author's use of a digital repository.  They can often be a good way to increase exposure and access to your publications.  Three common types of digital repositories are institution repositories, disciplinary repositories, and commercial repositories.

Posting on an institutional repository is generally limited to those affiliated with the institution.  Often, documents may remain in the database even if the author has moved to another institution. 

Disciplinary repositories encourage scholars from multiple institutions to post research in some specific field of scholarship.

A few repositories, such as Academia.edu, ResearchGate, or SciHub, gather posts from any discipline or academic institution.  These are each, in some sense, commercial endeavors because they each have direct or indirect approaches to generating income.  They combine features of digital repositories with social networking and with the types of metrics that assess the importance of a publication by it’s “citedness.”  While these systems have attracted many scholars, they have also generated controversy.  A useful guide to using these systems to increase your scholarly visibility is “The 30-Day Impact Challenge” by Stacy Konkiel.  An introduction to the potential hazards is provided by “Understanding Academia.edu and ResearchGate,” part of a guide posted by the Libraries at the University of Oklahoma, or by Jefferson Pooley, “Metrics Mania: The case against Academia.edu,” The Chronicle of Higher Education (January 7, 2018).

Libraries often take an active role in the development of digital repositories and even as publishers of open access books and journals.  A national organization, the Scholarly Publishing and Academic Resources Coalition (SPARC) began in 1998 with an emphasis on libraries and open access publishing, and has since broadened to the other concerns of open scholarship.