References:
XML Schema, a brief introduction
http://lucas.ucs.ed.ac.uk/xml-schema/

W3Schools - XML Schema Tutorial
http://www.w3schools.com/schema/default.asp

XML Schemas - Tutorial and Best Practices
http://www.xfront.com/
http://www.xfront.com/xml-schema.html
http://www.xfront.com/BestPracticesHomepage.html

XML Schema - Structures Quick Reference
http://www.xml.dvint.com/docs/SchemaStructuresQR-2.pdf
http://dret.net/lectures/xml-fall06/xsd-quickref.pdf
http://www.xml.com/pub/a/2000/11/29/...cturesref.html

W3C XML Schema
http://www.w3.org/XML/Schema

W3C XML Schema Part 0: Primer Second Edition
http://www.w3.org/TR/xmlschema-0/

W3C XML Schema Part 1: Structures Second Edition
http://www.w3.org/TR/xmlschema-1/

W3C XML Schema Part 2: Datatypes Second Edition
http://www.w3.org/TR/xmlschema-2/