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Short for XML Schema Definition, a way to describe and validate data in an XML environment. (A schema is a model for describing the structure of information.) XSD is a recommendation of the W3C.
XSD has advantages over earlier XML schema languages, such as DTD. Because XSD is written in XML, there is no need for a parser. XSD defines a richer set of data types such as booleans, numbers, dates and times, and currencies -- which is invaluable for e-commerce applications. DTDs, on the other hand, express data types as explicit enumerations, which makes validation much more difficult and less accurate.
More important, XSD makes it easier to validate documents based on namespaces (used to qualify element and attributes names by associating them with namespaces identified by URI references. Namespaces prevent identically custom-named tags that may be used in different XML documents from being read the same way), something DTDs cannot do.
XSD is defined in the W3C's XML Schema Working Group Working Draft published on May 6, 1999.
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 Understanding XML Schemas 
Article explores what schemas are, what validity means, how schemas differ from DTDs, and what new functionality will be gained from adopting them.
Using W3C XML Schema
This article is an introduction to using W3C XML Schemas, and also includes a comprehensive reference to the Schema data types and structures.
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