Question

When is Latex Used? It's used in OS Eqn Editor, but when else is it used??

Answer 1

It's use to write documents, articles, reports somewhat similar function as MS Word

Answer 2

Outside of the internet, LaTeX (and TeX) has found a use as a markup language to typeset your documents and your equations the way you like.

It is like the alternative to MS Word, but I think LaTeX isn't WYSIWYG; a LaTeX source document looks more like what you see when you right click a web page and select `view source', a bunch of markup tags and stuff.