MediaWiki uses a subset of AMS-LaTeX markup, a superset of LaTeX markup which is in turn a superset of TeX markup, for mathematical formulae. It generates either PNG images or simple HTML markup, depending on user preferences and the complexity of the expression. In the future, as more browsers become smarter, it will be able to generate enhanced HTML or even MathML in many cases.
To add a magic word, you need to first be in source editing mode on an article.
You must use the following format: <math> math to be displayed </math>.