This page describes the BibTeX file format, .bib. BibTeX was originally devised by Oren Patashnik and Leslie Lamport in 1985, to be used with LaTeX, but can now be used with many word and text processing systems. It is "available under the same terms as Donald Knuth's TeX program".
The web site http://www.bibtex.org/ seems to be an authoritative source of information. The format is described at http://www.bibtex.org/Format/. BibTeX files are plain text files formatted according to the BibTeX specifications.
Detailed technical information at the TeX level is available at http://www.tex.ac.uk/tex-archive/bibliography/bibtex/base/bibtex.web.
BibTeX has a Wikipedia page at http://en.wikipedia.org/wiki/BibTeX. There does not appear to be an entry for BibTeX in the PRONOM database.
There were no new versions of BibTeX between 1988 and 2010, when version 0.99d was released.
BibTeX files can be converted "to anything" using Bib2X according to http://www.bibtex.org/Convert/.
A list of BibTeX style examples is at https://verbosus.com/bibtex-style-examples.html.
The file extension is usually .bib (sometimes .bibtex) and the MIME Type is application/x-bibtex.
LaTeX: A Document Preparation System by Leslie Lamport, 1986, Addison-Wesley.
There are many guides to BibTeX available on the web.