The Document Object Model (DOM)

The Document Object Model (DOM) is an application programming interface (API) for valid HTML and well-formed XML documents. It defines the logical structure of documents and the way a document is accessed and manipulated With DOM API, 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 DOM API.
Its a W3C specification, one important objective for the DOM is to provide a standard programming interface that can be used in a wide variety of environments and applications.

Reference:
http://www.w3.org/TR/DOM-Level-2-Core/introduction.html