IBM网站上给出的XML的介绍（9）不完全摘录

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Overview

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　　So far in this tutorial you've learned about the basic rules of XML documents; that's all well and good, but you need to define the elements you're going to use to represent data. You'll learn about two ways of doing that in this section.

• One method is to use a Document Type Definition, or DTD. A DTD defines the elements that can appear in an XML document, the order in which they can appear, how they can be nested inside each other, and other basic details of XML document structure. DTDs are part of the original XML specification and are very similar to SGML DTDs.
  
• The other method is to use an XML Schema. A schema can define all of the document structures that you can put in a DTD, and it can also define data types and more complicated rules than a DTD can. The W3C developed the XML Schema specification a couple of years after the original XML spec.