请前辈们帮忙解释下一下代码的含义

[unfish]

<!--html-->


<div id="elem" class="gddiv">
<div class="innerElem">
<ul>
<li><a href=""><img src="images/gd.jpg" /></a></li>
<li><a href=""><img src="images/gd2.jpg" /></a></li>
<li><a href=""><img src="images/gd.jpg" /></a></li>
<li><a href=""><img src="images/gd2.jpg" /></a></li>
<li><a href=""><img src="images/gd.jpg" /></a></li>
<li><a href=""><img src="images/gd2.jpg" /></a></li>
</ul>
</div>
</div>



<!--Javascript-->


function slide(id){
var timer;
var elem = document.getElementById(id);//搞不明白elem和id 的关系
var elem1 = elem.getElementsByTagName('ul')[0];
var elem2 = document.createElement('ul');
var div = elem.getElementsByTagName('div')[0];
this.Scroll = Scroll;
this.act = act;
function Scroll() {
if (elem.scrollLeft >= elem1.offsetWidth) {
elem.scrollLeft -= elem1.offsetWidth;
}
else {
elem.scrollLeft += 3;
}
}
function act() {
div.appendChild(elem2);
if (elem1.offsetWidth >= elem.offsetWidth) {
elem2.innerHTML = elem1.innerHTML;
timer = setInterval(this.Scroll, 80);
elem.onmouseover = function () {
clearInterval(timer);
}
elem.onmouseout = function () {
timer = setInterval(Scroll, 80);
}
}
}
}

$(".gddiv").each(function () {
var sli = new slide($(this).attr("id"));
sli.act();
});
</script>

[slwsss]

http://www.w3school.com.cn/jsref/met_doc_getelementbyid.asp

[usecf]

var elem = document.getElementById(id);//搞不明白elem和id 的关系 id相当于起了个名字 elem 相当于根据名字找到对应的元素对象，有了对象才能调用相应的方法