How to add a description to Widgets?

Here’s what you looking for:

class WP_Widget_Sitemap extends WP_Widget {

    function WP_Widget_Sitemap() {
        $widget_ops = array( 'classname' => 'widget_sitemap', 'description' => __( "This is the description" ) );
        $this->WP_Widget( 'sitemap', __('Site Map'), $widget_ops);
    }

    function widget() { ... }
    function form() { ... }
    function update() { ... }
...

See: http://codex.wordpress.org/Widgets_API#Developing_Widgets