title: What is the cross domain issue
body: <p>If I asked a stupid question here, Please don't laugh at me . </p>
  
  <p>I have being heard <code>Cross Domain Issue</code> many times, But not really get involved in it in real case. So I searched it in the google. But found many posts are talking about <code>The cross domain issue when make a ajax call</code>. Not found even a post to tell about what exactly cross domain issue is, and why the cross domain is not allowed? and more question is if I say the cross domain issue, does it mean I made a wrong ajax request to different domain? Any other cases would cause this issue? Thanks.</p>
  
  <p>The posts I read are</p>
  
  <p><a href="https://stackoverflow.com/questions/298745/how-do-i-send-a-cross-domain-post-request-via-javascript">How do I send a cross-domain POST request via JavaScript?</a></p>
  
  <p><a href="https://stackoverflow.com/questions/20035101/no-access-control-allow-origin-header-is-present-on-the-requested-resource-w">&quot;No &#39;Access-Control-Allow-Origin&#39; header is present on the requested resource&quot;</a></p>