What does this code mean? Why including JS like this??!!!

What does this code mean? Why including JS like this??!!!

  1. <script type="text/javascript">
  2.         window.jQuery || document.write("<script src='js/jquery-2.0.3.min.js'>"+"<"+"/script>");
  3.         </script>
Why not just

  1. <script src='js/jquery-2.0.3.min.js'></script>

Thanks