XML_DTD is able to parse a document type definition (DTD) file and validate XML documents according to this DTD. A DTD is the grammar of an XML application, it consist of rules that declare how tags have to be nested and which attributes may be used in the tags.
If you want to learn how to create a DTD, you should take a look at the W3C.
XML_DTD consits of two parts:
XML_DTD_Parser, which will parse a DTD file and return an XML_DTD_Tree object
XML_DTD_XmlValidator, which validates an XML document using a DTD file
If you validate an XML_Document using XML_DTD_XmlValidator, it will automatically parse the DTD file and create an XML_DTD_Tree from it.