document.onload
It gets fired prior to loading of images and other external content. document.onload event is fired before the window.onload.
window.onload
It gets fired when the complete page loads, which includes images, scripts, css, etc.
Example
Here’a an example to understand onload.
Live Demo
<html>
<head>
<title>JavaScript Animation</title>
<script>
<!--
var imgObj = null;
function init() {
imgObj = document.getElementById('myImage');
imgObj.style.position= 'relative';
imgObj.style.left = '0px';
}
function moveRight() {
imgObj.style.left = parseInt(imgObj.style.left) + 10 + 'px';
}
window.onload =init;
//-->
</script>
</head>
<body>
<form>
<img id="myImage" src="/images/html.gif" />
<p>Click button below to move the image to right</p>
<input type="button" value="Click Me" onclick="moveRight();" />
</form>
</body>
</html>