How to get Latex

In order to create documents with latex you need a text editor and a LaTeX compiler Text editor. Any text editor can be used. Notepad, Notepad++, BBEdit, WinEdit, Alphatk or any other. There are also some text editor designed for being use with LaTeX, e.g. Texmaker,Inlage, Kile, LEd. A LaTeX compiler. This is the software that compiles the text into a LaTeX format. The most well known compilers are: TeX Live. It works in different operating systems and contains all major related programs. MACTeX. It is like TeX Live for MAC systems. MiKTeX and proTeX. They for windows system. Personally, I suggest a combination of TeX Live with Tex Maker. Tex Live is a simple compiler with the most used libraries. Tex Maker is text editor linked to the compiler. It allows to compile and view the document within one window and using key shortcuts.

 Additional reading.
List of text editors.
Obtaining LaTeX.
More distributors.

What is LaTeX?

LaTeX is a document preparation system for high-quality typesetting. It is most often used for medium-to-large technical or scientific documents but it can be used for almost any form of publishing. LaTeX is not a word processor!

LaTeX is based on the idea that authors should be able to focus on the content of what they are writing without being distracted by its visual presentation. In preparing a LaTeX document, the author specifies the logical structure using familiar concepts such as chapter, section, table, figure, etc., and lets the LaTeX system worry about the presentation of these structures. It therefore encourages the separation of layout from content while still allowing manual typesetting adjustments where needed.

Additional reading:
http://www.latex-project.org/intro.html
http://en.wikipedia.org/wiki/LaTeX