Ivor Horton's Beginning Java™ 2, JDK™ 5th Edition

22.7. XML Schemas

Because of the limitations of DTDs that I mentioned earlier, the W3C has developed the XML Schema language for defining the content and structure of sets of XML documents, and this language is now a W3C standard. You use the XML Schema Definition language to create descriptions of particular kinds of XML documents in a similar manner to the way you use DTDs, and such descriptions are themselves referred to as XML Schemas and fulfill the same role as DTDs. The XML Schema language is itself defined in XML and is therefore implicitly extensible to support new capabilities when necessary. Because the XML Schema language enables you to specify the type and format of data within an XML document, it provides a way for you to define and create XML documents that are inherently more precise, and therefore safer than documents described by a DTD.

It's easy to get confused when you are working with XML Schemas. One primary source of confusion is the various levels of language definition you are involved with. At the top level, you have XML—everything you are working with in this context is defined in XML. At the next level you have the XML Schema Definition language—defined in XML of course—and you use this language to define an XML Schema, which is a specification for a set of XML documents. At the lowest level you define an XML document—such as a document describing a Sketcher sketch—and this document is defined according to the rules you have defined in your XML Schema ...