What is LaTeX?
LaTeX (pronounced Lay-Tech or Lah-Tech) was written by L. B. Lamport and was made from TeX, which was a computer program for creating documents. It seems to be used most often by high level science scholars because LaTeX can work with mathematical notation. But, LaTeX can be and is used by lots of people for creating top quality documents which can survive translation into multiple formats.
Why do we need LaTeX?
LaTeX allows you to be the author and the book designer, rather than just the author. LaTeX acts as typesetter who follows your commands as the book designer and author to create professional documents with relative ease. LaTeX allows you to control how your document looks and to understand why it looks the way it does. LaTeX, like any markup language, will also help you have a sense of organization, flow, and structure within your work. As interdisciplinary programs grow, technical-format requirements for publication increase. LaTeX offers a one step (or close to one step) solution for documents which need to be in multiple formats. LaTeX documents are all plaintext files. This means you can edit them with any editor, and transfer them to any computer system running LaTex and they will format exactly the same. This is unlike Microsoft Word and many other programs which will alter based on the system, version of software and system, and other variables. Nothing is hidden in LaTeX and nothing will go out of date. LaTeX will never have the gooblety-gook problems that converting documents from Word Perfect to Word to StarOffice incur. In LaTeX, users type the text of their documents along with the accompanying markup (also referred to as commands or control sequences, with control sequences being the proper TeX term). The markup identifies parts of the document by name, such as "title" and "section." LaTeX does the formatting by using the user's markup to guide its internal rules and external stylesheets for typesetting.
This page is drawn from Peter Flynn's "A Beginner's Guide to Typesetting with LaTex," http://www.ctan.org/tex-archive/info/beginlatex/beginlatex.pdf.