Menu items description? Custom Walker for wp_nav_menu()

You need a custom walker for the nav menu.

Basically, you add a parameter 'walker' to the wp_nav_menu() options and call an instance of an enhanced class:

wp_nav_menu(
    array (
        'menu'            => 'main-menu',
        'container'       => FALSE,
        'container_id'    => FALSE,
        'menu_class'      => '',
        'menu_id'         => FALSE,
        'depth'           => 1,
        'walker'          => new Description_Walker
    )
);

The class Description_Walker extends Walker_Nav_Menu and changes the function start_el( &$output, $item, $depth, $args ) to look for $item->description.

A basic example:

/**
 * Create HTML list of nav menu items.
 * Replacement for the native Walker, using the description.
 *
 * @see    https://wordpress.stackexchange.com/q/14037/
 * @author fuxia
 */
class Description_Walker extends Walker_Nav_Menu
{
    /**
     * Start the element output.
     *
     * @param  string $output Passed by reference. Used to append additional content.
     * @param  object $item   Menu item data object.
     * @param  int $depth     Depth of menu item. May be used for padding.
     * @param  array|object $args    Additional strings. Actually always an 
                                     instance of stdClass. But this is WordPress.
     * @return void
     */
    function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 )
    {
        $classes     = empty ( $item->classes ) ? array () : (array) $item->classes;

        $class_names = join(
            ' '
        ,   apply_filters(
                'nav_menu_css_class'
            ,   array_filter( $classes ), $item
            )
        );

        ! empty ( $class_names )
            and $class_names = ' class="'. esc_attr( $class_names ) . '"';

        $output .= "<li id='menu-item-$item->ID' $class_names>";

        $attributes  = '';

        ! empty( $item->attr_title )
            and $attributes .= ' title="'  . esc_attr( $item->attr_title ) .'"';
        ! empty( $item->target )
            and $attributes .= ' target="' . esc_attr( $item->target     ) .'"';
        ! empty( $item->xfn )
            and $attributes .= ' rel="'    . esc_attr( $item->xfn        ) .'"';
        ! empty( $item->url )
            and $attributes .= ' href="'   . esc_attr( $item->url        ) .'"';

        // insert description for top level elements only
        // you may change this
        $description = ( ! empty ( $item->description ) and 0 == $depth )
            ? '<small class="nav_desc">' . esc_attr( $item->description ) . '</small>' : '';

        $title = apply_filters( 'the_title', $item->title, $item->ID );

        $item_output = $args->before
            . "<a $attributes>"
            . $args->link_before
            . $title
            . '</a> '
            . $args->link_after
            . $description
            . $args->after;

        // Since $output is called by reference we don't need to return anything.
        $output .= apply_filters(
            'walker_nav_menu_start_el'
        ,   $item_output
        ,   $item
        ,   $depth
        ,   $args
        );
    }
}

Or, alternatively as @nevvermind commented, you could inherit all the functionalities of the parent's start_el function and just append the description to $output:

function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) 
{
    parent::start_el( $output, $item, $depth, $args );
    $output .= sprintf( 
        '<i>%s</i>', 
        esc_html( $item->description ) 
    );
}

Sample output:

Now enable the description field in wp-admin/nav-menus.php to get the ability to edit this field. If you don’t WP just trashes your complete post content into it.

Further reading:

• Associated bugs
• Similar example with different markup and formatting
• Enable description box in menu management screen programmatically
• Collect the item description for later usage

And that’s it.