A typical file, such as a term paper, will begin like this:
\documentstyle{article}
\begin{document}
This tells LaTeX the following things:
article. The article style may be used
for any kind of standard paper. An article is shorter than a
report.book style also exists. It is meant for real books. These
options control section headings and tables of contents and other
things.
\documentstyle[11pt]{article}
\documentstyle[twoside,doublespace,11pt]{report}
sets the document up as an 11-point, double-spaced, double-sided
report. Most of these options cause corresponding LaTeX
style files to be loaded (twoside.sty, doublespace.sty, etc.),
but some of them are built in to the document type (the 11pt
option is done this way). You generally won't care about this unless
you mistype one of the options or some optional package isn't
available with the current configuration (described later).
\documentstyle and the
\begin{document} commands is called the header area or
preamble. Various control parameters, such as the layout of the
title page (if you want one) go here. The actual text of your
document begins after the \begin{document}.