The preprint server arxiv is the oldest such repository. It started in 1991 covering physics, astronomy, mathematics & computer science. It has expanded its coverage into other mathematically based disciplines and houses over 1.8 million research articles.
This is a preprint collection of research articles on COVID-19 archived in the bioRxiv & medRxiv preprint collections.
Old Definition: Copy of an accepted research article before it appears in print. Articles used to be delayed by 3 to 6+ months before they appeared in print. The publisher would send preprint copies to the researchers who would then distribute them to fellow researchers.
New Definition: An electronic copy (normally a .pdf copy) of a research article before it is accepted for publication.
Open access websites that archive preprint research articles submitted by researchers. These servers are hosted by major academic & research organizations. The articles are archived at no cost to the authors.
Open access resource:
There is no charge for access to the articles. Most academic publishing requires charges by subscription or on an article basis.
The usual method for academic/scholarly publishing where one or more academic researchers review & judge your article before it is accepted for publication.
DOI: Digital Object Identifier is a unique number assigned to an article. This is a unique identifier so the article can be readily accessed on the internet.
Bibliometric data: This is statistical usage data collected on an article. It can include citations, downloads, page views, web mentions, etc. This statistical information is often used for various evaluative measures of productivity or usefulness.