“The Document Object Model (DOM) is an application programming interface (API) for HTML and XML documents. It defines the logical structure of documents and the way a document is accessed and manipulated. (...)
With the Document Object Model, programmers can build documents, navigate their structure, and add, modify, or delete elements and content. Anything found in an HTML or XML document can be accessed, changed, deleted, or added using the Document Object Model (...).”
(quoted from the DOM Level 1.0 specification)
For more information see the Wikipedia entry on the Document Object Model and the W3C DOM website.
Good documentation on the DOM is here: http://www.brainjar.com/dhtml/intro/ and http://www.brainjar.com/dhtml/events/