TeX is a high quality typesetting program offering extensive desktop publishing features and automation, such as numbering and cross-referencing, tables and figures, detailed page layout, bibliographies, and indexing. Also, TeX/LaTeX is the only VIABLE tool for creating high quality documents that contain mathematics, physics, chemistry, biology, and engineering notations. Furthermore, TeX/LaTeX documents can be easily converted into output that is ADA/508 compliant (e.g. MathML). Among many other applications, LaTeX can be also used for professional music typesetting. Here is one example of such use. The corresponding source code is in ascii and contains only markup language commands.
In contrast to most word processors, where one sees the document more or less as it will look when printed, LaTeX focuses on the meaning of what is being written without distractions by the visual presentation of the information. Finally, Open Source TeX/LaTeX is a professional typesetting and publishing tool (used by major publishing houses) that is free to use and/or to modify.
Mac users can go directly to the section 9 of the above tutorial:
Download and Installation instructions for Mac platform,
while Windows users are directed to section 10:
Download and Installation instructions for Windows platform,
Initially, the additional downloading options in sections 9.1-9.2, as well as in sections 10.1-10.2, of the above tutorial can skipped.
And here are Installation instructions for Linux platform.