I'm pretty new to XML myself, but basically a DTD is a "Document Type Definition", which defines data storage and description protocols and parameters for a particular implementation of XML. In other words, it tells you how to read a given XML document, and it tells a parser how to parse it (more or less).

Here's a decent link that might be useful getting you started:

http://www.w3schools.com/dtd/default.asp

Hope that helps.

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