Its a basic popup javascript.
Code:
<script type="text/javascript">
<!--
function MM_openBrWindow(theURL,winName,features) { //v2.0
window.open(theURL,winName,features);
}
//-->
</script>
There are lots of newer and more sophisticated methods out there now. Try googling popup javascripts. One in particular that I have been seeing a lot lately is called lightbox 2.0
Best of luck,
Zygote