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,
gives a more complete description. 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.
There is a Unix BibTeX man page in the web2c package (see TeX systems). Any copy you may find of a man page written in 1985 (before "BibTeXing" and "Designing BibTeX Styles" appeared) is obsolete, and should be thrown away.