BibTeX, a program originally designed to produce bibliographies in conjunction with LaTeX, is explained in Section 4.3 and Appendix B of Leslie Lamport's LaTeX manual (see TeX-related books). The document "BibTeXing", contained in the file btxdoc.tex, expands on the chapter in Lamport's book. The LaTeX Companion (see TeX-related books) also has information on BibTeX and writing BibTeX style files.
The document "Designing BibTeX Styles", contained in the file
btxhak.tex, explains the postfix stack-based language used to write
BibTeX styles (.bst files). The file btxbst.doc is the template
for the four standard styles (plain, abbrv, alpha, unsrt). It
also contains their documentation.
The complete BibTeX documentation set (including the files above)
is available on CTAN.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=BibTeXing