Example from WordPress Codex for the Widget API with CSS classes

wordpress_widget_api_example_2.php

<?php
	/**
	 * Register widget with WordPress.
	 */
	function __construct() {
		parent::__construct(
			'foo_widget', // Base ID
			__( 'Widget Title', 'text_domain' ), // Name
			array( 
				'classname'	=> 'YOUR WIDGET CLASSES',
				'description'	=> __( 'A Foo Widget', 'text_domain' ), ) // Args
		);
	}