XML Schema, published as a W3C recommendation in May 2001, is one of several XML schema languages.
Because of confusion between XML Schema as a specific W3C specification, and the use of the same term to describe schema languages in general, some parts of the user community referred to this language as WXS, an initialism for W3C XML Schema, while others referred to it as XSD, an initialism for XML Schema Document — a document written in the XML Schema language, typically containing the "xsd" XML namespace prefix and stored with the ".xsd" filename extension.
As of April 5th 2012, the XML Schema Definition (XSD) 1.1 is a W3C Recommendation; W3C has adopted XSD as the preferred acronym.
What is an XML Schema?
The purpose of an XML Schema is to define the legal building blocks of an XML document, just like a DTD.
An XML Schema:
- defines elements that can appear in a document.
- defines attributes that can appear in a document.
- defines which elements are child elements.
- defines the order of child elements.
- defines the number of child elements.
- defines whether an element is empty or can include text.
- defines data types for elements and attributes.
- defines default and fixed values for elements and attributes.