Should you be interested in learning more about the DOM, there are a number of resources available to you online. Here's a brief list:
The official W3C DOM specifications, at http://www.w3.org/DOM/
Mozilla.org developer resources, at http://www.mozilla.org/docs/ and
http://www.mozilla.org/docs/web-developer/
DOM sample code at http://www.mozilla.org/docs/dom/samples/
An interesting article on transitioning from proprietary DOMs to the W3C standard, at http://sites.netscape.net/ekrockhome/standards.html
A structural (logical) view of the DOM, at http://www.xml.com/1999/07/dom/xml_dom.gif
An XML introduction to the DOM, at http://www.xml101.com/dom/