XML enables users to create documents that contain more specific information about content than ever before, adding a certain level of "intelligence" to the documents. XML documents also have a well-defined structure.
HTML has devolved from its original intent of defining structure. But even defining structure is not enough.

To fully exploit the potential of evolving technologies, we must define not only the document structure, but the actual content as well.

XML is a World Wide Web Consortium recommendation that was created by the XML Working Group and the XML Special Interest Group at W3C. The recommendation defines an XML document as a text document containing storage units called entities.
These entities may contain parsed or unparsed data. XML provides a mechanism to impose constraints on the storage layout and logical structure of a document. These constraints are known as the well-formedness and validity constraints.
We will discuss these constraints and the formulation of XML documents later in the course.