title: What is the difference between jQuery "this" and JavaScript "this"?
body: <p>I am reading a jQuery book and sometimes I see the example like:</p>
  
  <pre><code>$('img[alt]').each(function(){
    $(this).replaceWith('&lt;span&gt;' + $(this).attr('alt')+ '&lt;/span&gt;')
  }
  </code></pre>
  
  <p>and sometimes I see examples like:</p>
  
  <pre><code>$('*').attr('title', function(index, value) {
  return value + 'I am element ' + index + ' and my name is ' + this.id);
  });
  </code></pre>
  
  <p>so see sometimes it is <code>$(this)</code> and sometimes it is just <code>this</code></p>
  
  <p>What's the difference? How do I know which one to use and when?</p>