DTD: Document Type Definition. Describes the elements and attributes that an XML document can contain. The (old) way of defining XML applications, nowadays XSD (XML Schema Definition) is preferred.

sdphp_talks.dtd
<!-- 
     sdphp_talks.dtd
     version 0.1, JMC
-->
<!-- Element definitions -->
<!ELEMENT % sdphp_talks   'sdphp_talks   (title, talk*)'>

<!ELEMENT % talk    'talk    (title, date, speaker, url+)'>
<!ATTLIST talk id CDATA #REQUIRED>

<!ELEMENT % title   'title   CDATA #REQUIRED'>

<!ELEMENT % date    'date    CDATA #REQUIRED'>

<!ELEMENT % speaker 'speaker CDATA #REQUIRED'>

<!ELEMENT % url     'url     CDATA #REQUIRED'>
<!ATTLIST url type (download|www|ftp) 'www'>