title: I don't understand Async support in servlets 3.0 API
body: <p>I come from a Java SE background, and I did some servlets tutorials and read Head First JSP &amp; servlet. I'm reading the <a href="http://www.javaworld.com/javaworld/jw-02-2009/jw-02-servlet3.html" rel="noreferrer">JavaWorld.com</a> article about Async support now but I don't quite understand it.</p>
  
  <p>What's Async is simply about ? 
  What is the difference between Ajax and Servlet Async?</p>
  
  <p>P.S I have a PHP background with ajax and I know the concept, but I haven't tried it with java</p>