XML DTD(Document Type Definition)

The purpose of a DTD is to define the legal building blocks of an XML document. It defines the document structure with a list of legal elements. A DTD can be declared inline in your XML document, or as an external reference.

Internal DTD

This is an XML document with a Document Type Definition: 
<?xml version="1.0"?>
<!DOCTYPE note [
  <!ELEMENT note    (to,from,heading,body)>
  <!ELEMENT to      (#PCDATA)>
  <!ELEMENT from    (#PCDATA)>
  <!ELEMENT heading (#PCDATA)>
  <!ELEMENT body    (#PCDATA)>
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

The DTD is interpreted like this:
!ELEMENT note (in line 2) defines the element "note" as having four elements: "to,from,heading,body".
!ELEMENT to (in line 3) defines the "to" element  to be of the type "CDATA".
!ELEMENT from (in line 4) defines the "from" element to be of the type "CDATA"
and so on.....

External DTD

This is the same XML document with an external DTD:  
<?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

This is a copy of the file "note.dtd" containing the Document Type Definition:

<?xml version="1.0"?>
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>

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