Key Definitions
XML - extensible markup language is an open standard (W3C) that provides a data format and a data modeling language for defining data.
DTD - document type definition is the modeling mechanism for XML. It provides the rules for how XML data is defined and logically related.
Well-formed XML - an XML document in proper XML format, but with no structural conformance to a DTD (flat XML).
Valid XML - an XML document in proper XML format with a structural conformance to a DTD (structured XML).
XSLT - extensible stylesheet language for transforming XML documents into other XML documents.