XML Schema Dialects

Use XML syntax to define rules of an XML document
Supply facilities not available in DTDs, such as data types, inheritance, and range constraints
Use same syntax as XML documents
Reusable through facility of Namespaces
Various dialects of XML schemas are currently in use, including DDML, DCD, XSDL, XDR, and SOX
Each dialect is based on an XML schema proposal or W3C working draft

Unlike DTDs (Document Type Definitions), XML schemas use the same syntax as XML documents. They are like DTDs in that they define the rules of an XML document, including element names and data types, how elements can appear in combination, and which attributes are available for each element.

XML schemas supply additional semantic information, known as metadata, to the structure of XML documents, databases, and other similar organizations of data. They contain new features such as rich data types, inheritance, and extensibility that are not available in DTDs. XML schema components are reusable through the facility of XML Namespaces. Namespaces allow context to be given to element names, so they will be unique.

Some XML schema dialects currently in use are:

Data Definition Markup Language (DDML), formerly known as XSchema
Document Content Description for XML (DCD), which builds on a subset of XML-Data
XML Schema Definition Language (XSDL), the World Wide Wide Consortium (W3C) proposed standard. XSDL is specified in two working drafts, XML Schema Part 1: Structures and XML Schema Part 2: Datatypes. See also a companion document, XML Schema Part 0: Primer.
XML Data (Reduced) subset for Internet Explorer 5 (XDR), called "XML Schema" by Microsoft, provides roughly the same functionality as DCD, but in a somewhat different XML grammar, with core constructs similiar to those in XSDL.
Schema for Object-oriented XML (SOX), describes XML document structures in terms familiar to object-oriented software developers.

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