XSD

XML Schema Definition Language is a three part specification from the W3C that provides the capability to specify and constrain XML applications.

 XML Schema Part 1: Structures specifies the XML Schema definition language.  This language offers facilities for describing the structure and constraining the contents of XML 1.0 documents. The schema language is itself represented in XML 1.0.  It provides for the specification of a superset of the capabilities found in XML 1.0 document type definitions (DTDs).  The purpose of  XML Schema: Structures is to define the nature of XML schemas and their component parts, provide an inventory of XML markup constructs with which to represent schemas, and define the application of schemas to XML documents.

 XML Schema Part 2: Datatypes proposes facilities for defining datatypes to be used in XML Schemas and other XML specifications. The datatype language is also represented in XML 1.0.  It provides a superset of the capabilities found in XML 1.0 document type definitions (DTDs) for specifying datatypes on elements and attributes.

XML 1.0 defines limited facilities for applying datatypes to document content based on the capabilities of DTDs to assign types to elements and attributes. However, document authors, including authors of traditional  documents and those transporting  data in XML, often require a higher degree of type checking to ensure robustness in document understanding and data interchange.   XML Schema Part 2: Datatypes addresses this requirement.

In addition, a third part of the specification,  XML Schema Part 0: Primer has been provided by the working group.  The "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."  The intended audience of the XML Schema Primer "includes application developers whose programs read and write schema documents, and schema authors who need to know about the features of the language, especially features that provide functionality above and beyond what is provided by DTDs."

XML Schema became a W3C Recommendation on May 2, 2001.