W3C XML Schema Definition Language (XSD) 1.1 Part 1: Structures

This document specifies the XML Schema Definition Language,
which offers facilities for describing the structure and
constraining the contents of XML documents, including those which
exploit the XML Namespace facility. The schema language, which is
itself represented in an XML vocabulary and uses namespaces,
substantially reconstructs and considerably extends the
capabilities found in XML document type definitions (DTDs). This
specification depends on XML Schema Definition Language 1.1
Part 2: Datatypes.