This article is an introduction to using W3C XML Schemas, and also includes a comprehensive reference to the Schema datatypes and structures.
(Editor's note: this tutorial has been updated since its first publication in 2000, to reflect the finalization of W3C XML Schema as a Recommendation.)

• Introduction to DTD : An introduction to the XML Document Type Definition.
• DTD - XML Building Blocks : What XML building blocks are defined in a DTD.
• DTD Elements : How to define the elements of an XML document using DTD.
• DTD Attributes : How to define the legal attributes of XML elements using DTD.
• DTD Entities : How to define XML entities using DTD.
• DTD Validation : How to test for DTD errors while loading XML documents.
• DTD Examples : Some DTD examples.

James Tauber's comprehensive listing of DTDs, categorized by application, such as commerce, multimedia, education, and more.

The XML Bookmark Exchange Language, or XBEL, is an Internet "bookmarks" interchange format. It was designed by the Python XML Special Interest Group on the group's mailing list.
The original intent was to create an interesting, fun project which was both useful and would demonstrate the Python XML processing software which was being developed at the time. Mark Hammond contributed the original idea, and other members of the SIG chimed in to add support for their favorite browser features. After debate which ranged far afield from the original idea, compromises were reached which allow XBEL to be a useful language for describing bookmark data for a range of browsers, including the major browsers and a number of less widely used browsers.
At this time, the formal document type definition (DTD) was finalized and documentation was written. These are available online here.

XML Schema: Datatypes is part 2 of the specification of the XML Schema language. It defines facilities for defining datatypes to be used in XML Schemas as well as other XML specifications. The datatype language, which is itself represented in XML 1.0, provides a superset of the capabilities found in XML 1.0 document type definitions (DTDs) for specifying datatypes on elements and attributes.

XML Schema: Structures specifies the XML Schema definition language, which offers facilities for describing the structure and constraining the contents of XML 1.0 documents, including those which exploit the XML Namespace facility. The schema language, which is itself represented in XML 1.0 and uses namespaces, substantially reconstructs and considerably extends the capabilities found in XML 1.0 document type definitions (DTDs). This specification depends on XML Schema Part 2: Datatypes.

XML Schema Part 0: Primer is a non-normative document intended to provide an easily readable description of the XML Schema facilities, and is oriented towards quickly understanding how to create schemas using the XML Schema language. XML Schema Part 1: Structures and XML Schema Part 2: Datatypes provide the complete normative description of the XML Schema language. This primer describes the language features through numerous examples which are complemented by extensive references to the normative texts.