What is "dtd"

The Document Type Definition, or simply DTD, contains the rules used for creating XML documents and other markup languages. These rules define which tags and attributes of these can be used in the document and their valid values.

This feature was already used before the XML standard appeared. Since the SGML standard the DTD was used as a standard form of document validation of this language, but since 2001 it has been gradually replaced by the XML Schema (XSD).

Link to example of Dtds: Link