The difference between an absolute URL and a relative URL is that absolute URLs are the entire URL and relative URLs are only a portion of the URL. Examples:

Absolute:
  • <img src "http://www.yourdomain.org/images/picture.gif">
  • <img src "https://www.yourdomain.org/images/picture.gif">
  • <script type="text/javascript" src="http://www.yourdomain.org/js/script.js"></script>
  • <script type="text/javascript" src="https://www.yourdomain.org/js/script.js"></script>

Relative
  • <img src "../images/picture.gif">
  • <script type="text/javascript" src="../js/script.js"></script>

If you compare these three ways to code an image:
  • <img src "../images/picture.gif">
  • <img src "https://www.yourdomain.org/images/picture.gif">
  • <img src "http://www.yourdomain.org/images/picture.gif">

You'll notice that the first is relative, the second is absolute and secure (note the "https" instead of "http"), and the third is absolute and not secure. The first two will work fine but the last one will cause the error. The same is true for these three ways to code a script:
  • <script type="text/javascript" src="../js/script.js"></script>
  • <script type="text/javascript" src="https://www.yourdomain.org/js/script.js"></script>
  • <script type="text/javascript" src="http://www.yourdomain.org/js/script.js"></script>