The form tag defines the boundary of all form controls.
<form action="your-script-url"> ... the content of your form ... </form>
To use the standard form style sheet requires at least two steps.
<link rel="stylesheet" href="http://www.unimelb.edu.au/template-assets/07/styles/contentform.css" type="text/css" media="screen" />
<form action="your-script-url" class="contentform"> ... </form>
To enable validation for the form you need to:
<script language="JavaScript" type="text/javascript" src="http://www.unimelb.edu.au/template-assets/07/js/form_validate.js"></script>
validate to the form tag<form action="your-script-url" class="contentform validate"> ... </form>
More information on the form validation javascript.