There is no short answer; if you are interested in something specific, please let me know - maybe I will be able to answer and include a section on it in [w]Help:Formulas[/w].
As LaTeX is old and well documented, you can search for 'LaTeX' or 'LaTeX cheatsheet' to see some short examples; however, LaTeX is really about extension packages, so a definite dictionary is impossible. Even worse, mimeTeX supports only a subset.
[w]User:Tino[/w] - GMAT Club Support
Ask me about the [w]Tests[/w] and all tech issues. Let me know if you find something broken or ugly on this site.