I therefore take it that no-one who actually writes ever uses it.
LuaTeX is an extended version of pdfTeX using Lua as an embedded scripting language. The LuaTeX projects main objective is to provide an open and configurable variant of TeX while at the same time offering downward compatibility.
Edit: that's what I meant by "none of their links", the pdfTex page doesn't say what it does either!
I know what TeX is. I should know what luatex is because I've been to their website, read through some links and even read through the introduction to their manual, and still I don't know.
And just listen to yourself: if I want to know what luatex is I shouldn't find out on their own website, I should have to go to Wikipedia?!
Do you know what it is? You've managed to reply to me twice and in neither of those replies have you simply stated: "luatex is software that does x. You'd want to use to to do y."
pdfTex will take TeX input and output PDF (and apparently other file formats as well).
LuaTeX should do the same (being a drop-in replacement), the difference being that it can also use Lua scripts to control how the output is formatted (versus everything being written in TeX).
Ok, well, maybe I don't understand TeX at all, because I thought that's exactly what it did.
EDIT: Ahh, wait, I have it! When they say "extension" they mean "implementation"! They actually replace TeX as an engine. Is that right? So, luaTex is just an implementation of TeX written in Lua?
I think they might if they're at the luaTex website. How does it become interesting? Is it a secret?