Explain what is DTD (Document Type Declaration)? Mention what is the difference between CDATA and PCDATA in DTD?

A DTD means Document Type Definition (DTD) which defines the structure, legal elements and attributes of an XML document.
 PCDATA: A PCDATA is a Parsed Character Data. XML parsers usually parse all the text in an XML document.
 CDATA: While CDATA is an Unparsed Character Data, the term CDATA is used about text data that should not be parsed by the XML parser.