The Document Object Model (DOM) is an interface
specification maintained by the W3C DOM Workgroup that
defines an application independent mechanism to access,
parse, or update XML data. In simple terms it is a
hierarchical model that allows developers to manipulate XML
documents easily Any developer that has worked extensively
with XML should be able to discuss the concept and use of
DOM objects freely. Additionally, it is not unreasonable to
expect advanced candidates to thoroughly understand its
internal workings and be able to explain how DOM differs
from an event-based interface like SAX.

The DOM is a W3C (World Wide Web Consortium) standard.
The DOM defines a standard for accessing documents like XML 
and HTML:"The W3C Document Object Model (DOM) is a platform 
and language-neutral interface that allows programs and 
scripts to dynamically access and update the content, 
structure, and style of a document."
The DOM is separated into 3 different parts / levels:

Core DOM - standard model for any structured document 
XML DOM - standard model for XML documents 
HTML DOM - standard model for HTML documents 
The DOM defines the objects and properties of all document 
elements, and the methods (interface) to access them.



What is the XML DOM?
The XML DOM is:

A standard object model for XML 
A standard programming interface for XML 
Platform- and language-independent 
A W3C standard 
The XML DOM defines the objects and properties of all XML 
elements, and the methods (interface) to access them.

In other words:

The XML DOM is a standard for how to get, change, add, or 
delete XML elements.

In the DOM, everything in an XML document is a node


DOM Nodes
According to the DOM, everything in an XML document is a 
node.

The DOM says:

The entire document is a document node 
Every XML element is an element node 
The text in the XML elements are text nodes 
Every attribute is an attribute node 
Comments are comment nodes