bibtex covers (i) the BibTeX program written by Oren Patashnik, (ii) its
.bst style files, (iii) its widely used source representation of bibliography entries in
.bib files and the corresponding output representation in
.bbl files, and (iv) use of these in creating documents.
BibTeX was written for use with LaTeX where it is most widely used, but support for it exists in Plain TeX and ConTeXt.
For general questions about bibliographies and citations, use bibliographies instead.
A particular BibTeX style can be looked up here.