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What’s a DTD and why do I need one (or not)?

One of the keys to XML (and SGML) is the Document Type Definition (DTD), the specification for a particular class of documents. A DTD says what types of elements are meaningful for his type of document, and how they fit together. DTDs are important because they provide a basis for creating and managing whole groups of documents of the same type in a uniform way. This improves the quality of your information, because at least one source of error or confusion has been removed.

http://www.webreference.com/authoring/languages/xml/questions/dtd.html