The purpose of DTD is to define the legal building blocks of an XML documetn. It defines the documents structures with a list of legal elements.
DTD is an XML file:
With dtd, each of xml file can carry a description of its own format with it
Independent groups of ppl can agree to use a common dtd for interchanging data
To verify that the data
XML document are made up by: Elements, Tags, Attributes, Entities, PCDATA, CDATA