What exactly is LaTeX for?

“Many people discover LaTeX after years of struggling with wordprocessors and desktop publishing systems, and are amazed to find that TeX has been around for over 25 years and they hadn’t heard of it.”
Peter Flynn

LaTeX is a free document preparation system that enables you to create beautifully typeset pages. It implements a set of commands designed to control TeX, the typesetting engine developed by Donald E Knuth. LaTeX stores the information about your documents as plain text, thus avoiding the risk of vendor lock-in and ensuring that your documents will still be editable twenty years from now. LaTeX processes the plain text data and, with pdfTeX working in the background, generates PDF output of the highest typographic quality—perfect for viewing on-screen or printing on paper. LaTeX runs on many platforms and is included as standard with most Linux distributions. Ready-to-run LaTeX systems are also available for Windows and Mac OS X.
miktex.org, tug.org/mactex