LaTeX (1jun2009)
LaTeX is a set of TeX macro aimed to simplify the production and standardize the layout of the high quality documentations produced by TeX. TeX takes as input a text file, containing a specific markup language, and returns a formatted text. The great advantage of this document preparation system respect the much more diffused WYSIWYG editors, is that author is not troubled with document style and appearance, and can focus just on contents.

Typical compilation workflow:

First edit a text file with your favourite editor, for example:
example.tex
\NeedsTeXFormat{LaTeX2e} \usepackage{amsmath} \title{\LaTeX example} \documentclass[12pt]{article} \begin{document} \maketitle %\thispagestyle{empty} this is commented! Hello \LaTeXe\ and \TeX{}! Some math: \begin{align} E &= mc^2 \\ m &= \frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}} \end{align} \end{document}
Then obtain a .pdf by the calling sequence:
C:> latex example.tex
C:> dvips example.dvi
C:> ps2pdf example.ps

This is the traditional compilation workflow, which allows you to use .eps images and PStricks magic.