I want to be able to understand the standard function description that is provided for Wordpress functions. In particular, could someone explain the following example from here:

Usage

<?php wp_list_categories( $args ); ?> 

Default Usage

<?php $args = array(
    'show_option_all'    => ,
    'orderby'            => 'name',
    'order'              => 'ASC',
    'show_last_update'   => 0,
    'style'              => 'list',
    'show_count'         => 0,
    'hide_empty'         => 1,
    'use_desc_for_title' => 1,
    'child_of'           => 0,
    'feed'               => ,
    'feed_type'          => ,
    'feed_image'         => ,
    'exclude'            => ,
    'exclude_tree'       => ,
    'include'            => ,
    'current_category'   => 0,
    'hierarchical'       => true,
    'title_li'           => __( 'Categories' ),
    'number'             => NULL,
    'echo'               => 1,
    'depth'              => 0 ); ?>

I can guess most of it, but in particular I can't guess:

• What does blank after the comma mean? Empty string?
• What is the __?
• How do I call the function? Keyword like python, positional arguments or do I have to pass an array?
• Is there anything else about Wordpress function descriptions which isn't covered in this example?

Thanks,

Chris