If you are asking yourself what XBL (eXtensible Binding Language) is, in short it is an XML language proprietary to Gecko-based browsers, and is sort of like IE's proprietary DHTML Behaviors, but more powerful. If you are still confused, just skip this section, it is not relevant to normal web design unless you are an übergeek.
Let's look at this XBL document:

<?xml version="1.0"?> 
<bindings xmlns="">
<binding id="mybinding"> 
<handler event="click">
// how do we get the event?
alert(event.type) // "click"

When you click on an element bound with this binding, you'll get an alert saying "click". The variable event natively holds the event object whenever inside an XBL document.