This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
A Document Type Definition (DTD) is the definition of a specific type of electronic document. Specifically, it defines (1) a vocabulary of element names and attribute names, (2) guidelines concerning permitted relationships between elements, (3) specifications for required and optional items, and (4) a compliant XML-based structure in accord with the rules specified in SGML (ISO 8859:1986) and/or XML specifications. XML is a simplified version of SGML.
You can create a DTD in a simple text editor such as UltraEdit once you understand the rules.