Custom Sidebar and custom sidebar id tage help

Hello! I have a small issue, mostly with me not completely understanding how WordPress pulls the sidebar id’s.

<div id="sidebar">
   <ul>
      <?php
      if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar('custom-sidebar') ) :
      endif; ?>
      <div style="width: 2px; height:200px; background-color: #D6D6D6"></div>
   </ul>
</div>

This is the custom sidebar I made, just following this tutorial here:
http://www.tastyplacement.com/add-sidebar-wordpress

The problem with this code is that I have another sidebar, and both use the id sidebar.

Just a few side questions about sidebars:
1) When I registered this sidebar in the functions.php file, I have this code:

if ( function_exists('register_sidebar') ) {
register_sidebar(array(
'name' => 'Custom Sidebar',
'id' => 'custom-sidebar',
'description' => 'Appears as the custom sidebar on the left of the page',
'before_widget' => '<div style="height: 20px"></div><li id="%1$s" class="widget %2$s">',
'after_widget' => '</li>',
'before_title' => '<h2 class="widgettitle">',
'after_title' => '</h2>'

When it says ‘id’=> ‘custom-sidebar’ – what does that refer to? What is that useful for?

How can I make this sidebar have its own id apart from the already made sidebar id?

I need a separate one because the formatting will be different.

Sorry if this is confusing, and thank you for your help