TeX/LaTeX Auxilliary Reference file
AUX is a data file format used by LaTeX. LaTeX is a macro package which uses TeX typesetting language in its documents. AUX files contain information used for cross-referencing, and is also used to transport information from one compiler run to the next. AUX files can be opened by programs that run TeX, such as MacTeX and MiKTeX.
MIME: application/x-tex, application/x-latex, text/plain
Learn more about .AUX files:
Read a step-by-step tutorial of how to use LaTeX on Wikibooks.
Search on IT Knowledge Exchange for .AUX-related questions.