A system for typesetting text which allows one to cleanly display mathematical notation.
LaTeX is an overlay on top of the TeX? typesetting system.
You use it by writing a text file in the LaTeX syntax, and then compiling that into a .dvi file with a latex compiler. From there, you can compile it into a postscript or a pdf file.
For more information, see http://www.latex-project.org/