http://www.javacommerce.com/tutorial/xmlj/dtd.htm
This site describes what a DTD is and when to use an internal, external, or shared DTD.
Useful and helpful sub sites of this page include:
Elements - http://www.javacommerce.com/tutorial/xmlj/element.htm
Attributes - http://www.javacommerce.com/tutorial/xmlj/attribut.htm
Entities - http://www.javacommerce.com/tutorial/xmlj/entities.htm