PreprintA preprint is a draft of a scientific paper that has not yet been published in a peer-reviewed scientific journal.
Some preprints are great science -- some are mediocre -- some are nonsense. There is minimal pre-publication quality control, and the quality control comes from the fact that the papers are intended for publication in journals.
However, preprints published on websites have overtaken articles in scientific journals as the primary means by which many scientists keep track of current events. The peer review process typically takes several months to complete, by which time the articles may be out of date. In deciding a legitimacy of a preprint, most scientists have an internal filter which takes into account a number of factors including who wrote the preprint.
Preprints are generally not used in making hiring or promotion decisions in academia. Also, preprints tend not to be cited often. In citing references, most authors much prefer journal articles which are easier to find.
In recent years, preprints have started to become available online. The term e-print is sometimes used for preprints that are available electronically.
Online sources of preprints: