document.getElementsByTagName('div') returns an array of nodes (a HTMLCollection) whose HTML element is <div>. <!DOCTYPE html>
<html>
<head>
<meta content="text/html;charset=utf-8" http-equiv="Content-Type">
<title>getElementsByTagName</title>
<script type="text/javascript">
function main() {
try {
var node_list = document.getElementsByTagName('body');
node_list[0].innerHTML = "This is the body element";
} catch(e) {alert (e);}
}
</script>
</head>
<body onload='main();'>
</body>
</html>
document.getElementById('…') returns the (single) element, if it exists, whose attribute id has a given value.