What are classes and packages?

The main difference between LaTeX 2.09 and LaTeX2e is in the commands before \begin{document}.

In LaTeX 2.09, documents had styles, such as article or book, and options, such as twoside or epsfig. These were indicated by the \documentstyle command:

\documentstyle[<options>]{<style>}

For example, to specify a two-sided article with encapsulated PostScript figures, you said:

   \documentstyle[twoside,epsfig]{article}

However, there were two different types of document style option: built-in options such as twoside; and packages such as epsfig.sty. These were very different, since any LaTeX document style could use the epsfig package but only document styles which declared the twoside option could use that option.

To avoid this confusion, LaTeX2e differentiates between built-in options and packages. These are given by the new \documentclass and \usepackage commands:

\documentclass[<options>]{<class>}
\usepackage[<options>]{<packages>}

For example, to specify a two-sided article with encapsulated PostScript figures, you now write:

   \documentclass[twoside]{article}
   \usepackage{epsfig}

You can load more than one package with a single \usepackage command; for example, rather than writing:

   \usepackage{epsfig}
   \usepackage{multicol}

you can specify:

   \usepackage{epsfig,multicol}

Note that LaTeX2e still understands the LaTeX 2.09 \documentstyle command. This command causes LaTeX2e to enter LaTeX 2.09 compatibility mode, which is described in Section 4.

You should not, however, use the \documentstyle command for new documents because this compatibility mode is very slow and the new features of LaTeX2e are not available in this mode.

To help differentiate between classes and packages, document classes now end with .cls rather than .sty. Packages still end with .sty, since most LaTeX 2.09 packages work well with LaTeX2e.