TeX, LaTeX, pdfTeX, *TeX, are a typesetting system plus a collection of programs for producing publication-quality documents, often containing mathematical formulae. They are widely used in STEM fields.

With traditional word processors, text transformation is interactive — the user selects the third paragraph and make it boldface. With TeX, changes are made via markup commands that are embedded with the content in a text file. For instance, a new book chapter is started with the \chapter{…} command rather than with newlines and a number. In a separate step, the text file is processed (at which point all \chapter{…} commands are counted and turned into numbers, etc.) into the output document.

Both *TeX file formats and *TeX software are generally open, giving the user maximum flexibility and the opportunity to know all implementation details. That does not mean that a beautifully typeset result will be easy to produce. ☺ However, the system will be predictable, and the results, generally reproducible. In contrast, proprietary word processors are black boxes.