Preprints are versions of articles which are made available before they are peer-reviewed and published. The preprints are not peer-reviewed, edited, or typeset before being posted online. For a useful overview see this 2019 JISC report.
Preprints are usually made available on a preprint server by the author(s). Preprint servers are often dedicated to articles in a specific discipline or group of disciplines. Examples of preprint servers include:
('ArXiv' in the name of most of these servers is pronounced 'Archive')
Some publishers post preprints as part of their publication processes either to an existing preprint server or to a publisher owned platform.
There are also publisher owned platforms that authors can post to, e.g. SSRN
Note that not all preprints go on to be published in a peer reviewed journal or other venue.
There are several advantages to publishing a preprint:
Preprints can be useful to find out about current research. However, they have not been scrutinised or critiqued in a peer review process.
Some preprint services will undertake some moderation of content, for example if the preprint is in the wrong format or to prevent posting of offensive content, but they do not undertake peer review. You must therefore evaluate preprints while bearing in mind that there has been no review. There may also be major differences between a pre-print and any final published version which will come later.
MedRxiv gives the following warning:
"Caution: Preprints are preliminary reports of work that have not been certified by peer review. They should not be relied on to guide clinical practice or health-related behavior and should not be reported in news media as established information."
If you find an interesting preprint, you can check to see if it has gone on to be published. There may be a later peer-reviewed version available, for example in a journal.
However, many preprints do not go on to be published.
There has been an increase in preprints being made available in response to the Covid-19 pandemic.
This has prompted discussions of the benefits and risks associated with the rapid dissemination of unrefereed preprints.
Below are some examples:
Between fast science and fake news: Preprint servers are political - a post from the LSE Impact Blong
Preprints Involving Medical Research—Do the Benefits Outweigh the Challenges? - an editorial appearing in JAMA.