The XML Schema language is defined in three parts:

• A primer, located at w3.org/TR/xmlschema-0, that gives an introduction to XML schema documents and what they're designed to do;
• A standard for document structures, located at w3.org/TR/xmlschema-1, that illustrates how to define the structure of XML documents;
• A standard for data types, located at w3.org/TR/xmlschema-2, that defines some common data types and rules for creating new ones.

This tutorial discussed schemas briefly in Defining document content; if you want the complete details on all the things you can do with XML schemas, the primer is the best place to start.