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
\documentclass or \usepackage 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would get a warning that their copy of article was out of
date.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 \listfiles command. The phrase
Standard LaTeX must not be used in the identification
banner of any file other than those in the standard LaTeX
distribution.