DOM is not really a "new thing", it's an attempt to a) standardize the
different "Dynamic HTML" APIs in the 4.0 versions of the major HTML
browsers so that (one hopes) the 5.0 versions will allow much greater
interoperability, b)to extend the idea of a standard API to XML browsers
and editors, and c) to cover multiple languages, not just ECMAScript.
The DOM is an abstract object model for XML and HTML documents, i.e., a
description of a set of interfaces (Element, Attribute, Text, etc.) and
methods that allow XML and HTML documents to be constructed, navigated, and
modified via a common API across all products that support it. It is not
intended to support full-blown SGML, e.g., there's no marked section
interface in the API.
For the moment the DOM API just describes a generic XML document or
dataset, but they no doubt be extended some day to describe "power tools"
for the manipulation of XLL and XSL information associated with an XML
document.
I hope this helps. See the DOM page at http://www.w3.org/DOM/ for more
information on the current draft of the spec, the mailing list, etc.