The Preamble is where the overall construction of your document is listed. As such, it's best to write your preamble, put in some sample text, save, and then check to make sure it works. That way you'll know that the overall structure is solid before you begin actually writing and involving more complications.
The Preamble is where your document is defined. Any changes which affect your entire document go in the preamble, immediately after the \documentclass line and before the \begin{document} line.
The commands for the packages your document will use should go in the
preamble. For example, commands like:
\usepackage[latin1]{inputenc}
\usepackage{palatino,url}
The \renewcommand allows you to change the name of
the environment to suite your needs. For instance, the abstract
environment can be switched to Summary using the
\renewcommand command in this
syntax:
\renewcommand{\abstractname}{Summary}
Notice that the name of the command you are changing or renewing goes in the first set of curly braces and the new value goes in the second set of curly braces. The overall environment name stays the same when you use it later on in the document, but because it has been changed in the preamble, LaTeX will recognize the change and automatically substitute the item in the second curly braces for the original in the first curly braces.
A sample preamble could look like this:
\documentclass[12pt]{article}
\usepackage{url,graphicx,tabularx,array}
\usepackage[margin=.5in,nohead,nofoot]{geometry}
\begin{document}
The preamble ends once the document begins with the \begin{document} command.