The first thing a class or package file does is identify itself. Package files do this as follows:
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{<package>}[<date> <other information>]
For example:
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{latexsym}[1994/06/01 Standard LaTeX package]
Class files do this as follows:
\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{<class-name>}[<date> <other information>]
For example:
\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{article}[1994/06/01 Standard LaTeX class]
The date should be given in the form `YYYY/MM/DD'. This
date is checked whenever a user specifies a date in their
| or | command. For example, if you
wrote:
\documentclass{article}[1995/12/23]
then users at a different location
would get a warning that their copy of |article| was out of
date.
The description of a class is displayed when the class is used. The description of a package is put into the log file. These descriptions are also displayed by the || command.