title: Event listener vs subscriber in Symfony
body: <p>I get to know from documentation that subscriber already knows what should it subscribe to, but now I am in doubt that when defining listener we provide exact event when it should be fired like this </p>
  
  <pre><code>kernel.listener.my_listener:
      class: myBundle\EventListener\MyListener
      arguments:
          - @doctrine.orm.entity_manager
          - @twig
      tags:
          - { name: kernel.event_listener, event: kernel.request, method: myMethod}
  </code></pre>
  
  <p>So the listener exactly knows when to fire off
  But in case of subscriber we register it like this</p>
  
  <pre><code>kernel.listener.my_listener:
      class: myBundle\EventListener\myListener
      arguments: ["@service_container"]
      tags:
      - { name: kernel.event_subscriber }
  </code></pre>
  
  <p>Now, my question is how would subscriber know when to trigger or does it get triggered on every event??</p>