This is not a technical article on using TeX (i.e, TeX installation or programming). Instead, it offers some background information for people who work in STM (scientific, technical and medical) publishing and aims to provide an easy-to-follow explanation by addressing the question “what is TeX?”—and, hopefully, demystifies some confusing terminology. My objective is, quite simply, to offer an introduction to TeX-based software for new, or early-career, STM publishing staff—especially those working in production (print or digital). Just by way of a very brief bio, as in “am I qualified to write this”: I’m writing this piece based on my 20+ years of experience of STM publishing, having worked in senior editorial positions through to technical production and programming roles. In addition, over the last few years I have spent a great deal of time building and compiling practically every TeX engine from its original source code, together with creating my own custom TeX installation to explore the potential of production automation through modern TeX-based software.