A DTD is the definition of the allowable elements, attributes, and other document pieces of an XML document.
The DITA DTDs are (base) topic, concept, task, reference, map, and bookmap.
Example (concept DTD)
<!-- ============================================================= -->
<!-- HEADER -->
<!-- ============================================================= -->
<!-- MODULE: DITA Concept DTD -->
<!-- VERSION: 1.0.1 -->
<!-- DATE: November 2005 -->
<!-- -->
<!-- ============================================================= -->
<!-- ============================================================= -->
<!-- PUBLIC DOCUMENT TYPE DEFINITION -->
<!-- TYPICAL INVOCATION -->
<!-- -->
<!-- Refer to this file by the following public identifier or an
appropriate system identifier
PUBLIC "-//OASIS//DTD DITA Concept//EN"
Delivered as file "concept.dtd" -->
<!-- ============================================================= -->
<!-- SYSTEM: Darwin Information Typing Architecture (DITA) -->
<!-- -->
<!-- PURPOSE: DTD to describe DITA concepts -->
<!-- -->
<!-- ORIGINAL CREATION DATE: -->
<!-- March 2001 -->
<!-- -->
<!-- (C) Copyright OASIS Open 2005. -->
<!-- (C) Copyright IBM Corporation 2001, 2004. -->
<!-- All Rights Reserved. -->
<!-- ============================================================= -->
<!-- ============================================================= -->
<!-- DOMAIN ENTITY DECLARATIONS -->
<!-- ============================================================= -->
<!ENTITY % ui-d-dec PUBLIC
"-//OASIS//ENTITIES DITA User Interface Domain//EN"
"uiDomain.ent" >