The Tags Make the Document

The use of tags are known as an XML Application.  An XML Application is the application of XML to a particular problem.  It is not software code that uses XML nor is it a program written in XML.
 

The tags in an XML document describes the document’s structure as well as semantics.

XML enforces strict form.  All open tags must be closed for the document to be a well formatted XML document.  This is a deviation from standard HTML that does not enforce strict structure in its tags.

To further define the structure of documents, an optional document type definition (DTD) may be included.  All legal markup as well as where and how the markup may be used is included in the DTD.