A schema specifies the structure of an XML document and constraints on its content.
While XML is the meta-language that defines the rules for defining tag languages, a schema is a formal specification of the grammar for one tag language. A schema is useful for validating the document content—to determine whether a document is a valid instance of the grammar expressed by that schema—and for describing your grammar for use by others.
See these topics for more information on authoring XML Schemas: