解释一段javascript的程序

<div id="img" style="position:absolute;"> <img border="0" src="file:///E|/标志/test2.gif"
width="300" height="200"></div> //看来是关于图片的特效

<SCRIPT LANGUAGE="JavaScript">
<!-- Begin
var xPos = 20;
var yPos = document.body.clientHeight;
var step = 2;
var delay = 30;
var height = 0;

var Hoffset = 0;
var Woffset = 0;
var yon = 0;
var xon = 0;
var pause = true;
var interval;
img.style.top = yPos;
function changePos() {
width = document.body.clientWidth;
height = document.body.clientHeight;
Hoffset = img.offsetHeight;
Woffset = img.offsetWidth;
img.style.left = xPos + document.body.scrollLeft;
img.style.top = yPos + document.body.scrollTop; //看来是要图片滚动
if (yon) {
yPos = yPos + step;
}
else {
yPos = yPos - step;
}
if (y