TeX is a system for producing publication quality documents.
LaTeX is a free, user-friendly program for converting plain text to TeX. An author enters content into LaTeX and LaTeX provides the formatting and document design. Preparing LaTeX documents requires adding markup symbols to indicate section headings, citations and mathematical symbols.
LyX is an open-source editor that facilitates the insertion of markup symbols into LaTeX files for those who do not want to memorize these symbols. This open-source editor is available for Windows, Macintosh and Linux.
BibTeX is the citation management system used with LaTeX.