DynamicDOM is an extension of the W3C DOM (Document Object Model) that adds dynamism to XML. For example, it allows you to define rules so that when you change, say, the TotalIncome node, the IncomeTax node updates automatically.
Just as C++ adds code to the C struct to create a class, DynamicDOM adds intelligence, in the form of rules and code, to the DOM to expand its capabilities beyond parsing XML documents to processing and enforcing business rules. Your XML document, then, becomes dynamic in the sense that it contains rules and code in addition to data, and updates itself automatically based predefined rules.