Edit in JSFiddle

window.addEvent('domready', function(){
  $('feedback').setStyle('opacity', 0);

  $$('input, textarea').addEvent('focus', function(){
     $$('input, textarea').removeClass('focused');
     $(this).addClass('focused');
  });

  // 初始化helptext 数组,存储帮助文本
  var helptext = [];
  //帮助内容
  helptext['hName'] = 'Enter your first and last name';
  helptext['hEmail'] = 'Enter a valid email address';
  helptext['hWebsite']='Enter a valid website url';
  helptext['hComment'] = 'Leave your feedback';
  // 给 span.help 添加事件
  $$('.help').addEvents({
      'mouseover' : function() {
        $('feedback').tween('opacity', 1);
        // 获得 span's 的 ID
        spanID = $(this).get('id');
        // 设置 feedback div 的内容
        //使用spanID 索引数组
        $('feedback').set('html', helptext[spanID]);
    },
    'mouseout' : function() {
        $('feedback').tween('opacity', 0);
    }
  });
   
});
<form id="commentform" action="" method="get">
    <p>
        <label for="name">Name :  <span class="help" id="hName">(?)</span></label>
        <input id="name" name="name" tabindex="1" type="text">
    </p>
    <p>
        <label for="email">Email :<span class="help" id="hEmail">(?)</span> </label>
        <input id="email" name="email" tabindex="2"  type="text">
    </p>
    <p>
        <label for="url">Website :<span class="help" id="hWebsite">(?)</span></label>
        <input id="url" name="url" tabindex="3"  type="text">
    </p>
    <p>
      <label for="comment">Comment :<span class="help" id="hComment">(?)</span></label>
        <textarea id="comment" name="comment" cols="" rows="" tabindex="4"></textarea>
    </p>
    <p align="center">
        <input name="submit" type="button" tabindex="5" value="Submit">
    </p>
</form>
<!-- 拥有用户反馈 -->
<div id="feedback">&nbsp;</div>
<!--
body{
    font-family: Verdana, Geneva, sans-serif;
    font-size: 12px;
}
#commentform{
    border: 1px solid #cac9b9;
    padding: 5px;
    width: 450px;
}
#commentform p {
margin-bottom: 5px;    
}
#commentform label {
    float: left;
    width: 90px;
}
#commentform input,#commentform textarea {
border: 1px solid #cac9b9; background: #fff;
}
#commentform textarea{
    width: 200px;
}
#commentform .focused {
    border:1px solid #888;
    background-color: #ffffaa;
    /*css3 outline*/
    outline: 2px solid #ffffaa; 
}
#feedback {
    width:200px;
    position:absolute;
    top:15px;
    left:315px;
    border:1px solid #f90;
    background-color:#ffc;
    padding:5px;
    font:bold 12px Verdana, Geneva, sans-serif;
}
.help {
    color:#009;
    font:bold 12px Arial, Helvetica, sans-serif;
    cursor:pointer;
}
-->