LaTeX is a document preparation system. It is based on TeX, a typesetting system. It is a popular tool for mathematical documents as it provide robust support for mathematical notation. This guide is meant as an introduction to the tool and to provide useful information and links to Latex resources.
The TeX Stack Exchange is a popular and very useful question-and-answer forum for users of TeX and related systems.
If you are using TeXStudio or TeXMaker, or TeXworks you also need to download a distribution system also. Below are the most commonly used systems and what operating systems work with each.