<script>
function getHTML()
{
var url = 'http://yourserver/getSomeHTML.php';
var pars = 'someParameter=ABC';
var myAjax = new Ajax.Updater( 'placeholder', url, { method: 'get', parameters: pars });
}
</script>
<input type=button value=GetHtml onclick="getHTML()">
<div id="placeholder"></div>