TeX is the computer type-setting system written by Donald E. Knuth, and it has heralded a revolution in the quality of the vast majority of published mathematics.
However, most people use LaTeX, which is built on top of it.
Also exists mimetex, which is a lightweight clone of (a subset of) LaTeX, aimed at producing formula rendering for web pages and the like. It does not depend on having TeX or its fonts available.