LaTeX is a programatic typesetting system, or as the LaTeX web site puts it: "a document preparation system".
LaTeX is the de-facto typesetting system used by many technical and scientific publishers.
LaTeX is a language which for an author new to it adds to the burden of writing her paper, but the benefits are worth it. As a language it is most akin to a markup language like XML.
LaTeX's primary strengths are: