Every Window object has a document property that refers to a Document object. The Document object is arguably more important than the Window object itself: while the Window represents the browser window, the Document object represents the HTML document that is displayed in that window. The Document object has various properties that refer to other objects which allow access to and modification of document content. The way that document content is accessed and modified is called the document object model, or DOM, and there are several DOMs in existence:

Legacy DOM

The original legacy document object model evolved along with early versions of the JavaScript language. It is well supported by all browsers, but allows access only to certain key portions of documents, such as forms, form elements, and images.

W3C DOM

This document object model allows access and modification of all document content and is standardized by the World Wide Web Consortium (W3C). It is at least partially supported by Netscape 6 and later, Internet Explorer 5 and later, and other modern browsers. The W3C DOM is not closely compatible with the IE 4 DOM, but it does standardize many of the legacy features of the original DOM. This book covers the core features of the standard, and presents a simplified subset of the DOM relevant for JavaScript programmers working with HTML documents. You can find complete coverage in JavaScript: The Definitive Guide.

IE 4 DOM