window.onload = function() {                      //网页加载完执行该函数
        var oUl1 = document.getElementById("ul1"); //获取id为ul1的元素存入oUl1
        var aLi = oUl1.getElementsByTagName("li"); //获取oUl1中标签为li的元素
        var oDiv = document.getElementById("tab-list");//获取id为tab-list的元素存入oDiv
        var aDiv = oDiv.getElementsByTagName("div");  //获取oDIv中标签为div的元素
        for(var i = 0; i < aLi.length; i++) {        //循环aLi
            aLi[i].index = i;                        //aLi的下标赋值为i
            aLi[i].onmouseover = function() {        //滑动触发函数
                for(var i = 0; i < aLi.length; i++) {
                    aLi[i].className = "";            //循环为aLi添加class
                }
                this.className = "active";             //当前对象的class名为active
                for(var j = 0; j < aDiv.length; j++) {
                    aDiv[j].className = "hide";         //循环为aDiv添加类名为hide
                }
                aDiv[this.index].className = "show";   //aDiv的class=“show”
            }        
        }
    }

http://blog.csdn.net/kuangruike/article/details/50848585   这是对aLi[i].index = i;的解释