What is an XML Schema Definition file?

An XML Schema Definition (XSD) describes the structure of an XML document.

An XML Schema defines the building blocks of the XML document. An XSD describes the elements and attributes that can appear in a document, the data types for elements and attributes, the number of (and order of) child elements, as well as default and fixed values for elements and attributes.

XSD is fully recommended by W3C consortium as a standard for defining an XML document, and has replaced the use of Document Type Definition (DTD) files. For more information on XSD, please refer to the W3C consortium web site at http://www.w3.org.