XML Schema Specification - http://www.w3.org/XML/Schema
Using W3C XML Schema Tutorial: http://www.xml.com/pub/a/2000/11/29/schemas/part1.html - brief
XML Schema Tutorial - http://www.w3schools.com/schema/default.asp - more comprehensive
a complete tutorial and other resources available at http://www.xfront.com/files/xml-schema.html