\documentclass[12pt]{article}which determines the font size and document class - the type of document you're writing. Each class has an associated
*.cls
file in the
system directories which is read in at start-up time. Other options accepted by
article include 10pt, 11pt and twocolumn.
Then you'll probably need to load in extra macros with the
\usepackage
command. Each package has an associated
*.sty
file in the system directories. The packages inherit the
options from the \documentclass
line and can be given others of
their own. E.g.
\usepackage[dvips]{graphicx} \usepackage[dvips]{color}tells LATEX that you want to use the extra graphicx and color macros, and that you are going to use dvips to convert the resulting file to postscript. You can shorten this to
\usepackage[dvips]{graphicx,color}