Working with XML Schemas
To define the structure of an XML document, as well as its element relationships, data types, and content constraints, you use a document type definition (DTD) or XML Schema definition language (XSD) schema. Although an XML document is considered to be well-formed if it meets all the syntactical requirements defined by the World Wide Web Consortium (W3C) Extensible Markup Language (XML) 1.0 Recommendation, it is not considered valid unless it is both well-formed and conforms to the constraints defined by its DTD or schema. Therefore, although all valid XML documents are well-formed, not all well-formed XML documents are valid.
For more information about XML, see the W3C XML 1.0 Recommendation. For more information about XML Schema, see the W3C XML Schema Part 1: Structures Recommendation and the W3C XML Schema Part 2: Datatypes Recommendation recommendations.