Control Blocks

What is a control block?:

            <ul id="Menu1">
                   <% control Menu(1) %>
                <li class="$LinkingMode"><a href="$Link" title="Go to the &quot;{$Title}&quot; page">$MenuTitle</a></li>
                   <% end_control %>
            </ul>    

and.. 

<% if Menu(2) %>
  <ul id="Menu2">
    <% control Menu(2) %>
      <li class="$LinkingMode"><a href="$Link" title="Go to the &quot;{$Title}&quot; page">$MenuTitle</a></li>
    <% end_control %>
  </ul>
<% end_if %>