Style and Customize the Tag Cloud in WordPress

To change the appearance of the tag cloud, use the following snippet. A full reference of the options is available at this link:

// Tag cloud custom
function style_tags($args) {
    $args = array(
         'largest'    => '10',
         'smallest'   => '10',
         'format'     => 'list',
         );
    return $args;
}
add_filter('widget_tag_cloud_args', 'style_tags');

For more on tags and taxonomies, see Taxonomies in WordPress.