A formalized description of the structure of an XML document. XML schemas serve the same purpose as DTDs but are much more powerful and flexible. For example, a single document may use namespaces to take element definitions from more than one schema, with each element being validated against the appropriate schema. Schemas also allow element content to be restricted to specified data types (string, boolean, decimal number, etc.). Although the World Wide Web Consortium has issued a specification for XML schemas, other schema languages are also used.

http://www.w3.org/XML/Schema The W3C’s XML schema page