XSD can be used to express a set of rules to which an XML document must conform in order to be considered 'valid' according to that schema.
In case of web service all information about the web service is can be found inside its WSDL (Web Services Description Language) of a web service. WSDL (similar to XSD) is an XML-based interface description language that is used for describing the functionality offered by a web service. Check below links
Web Services Description Language
Check below link for creating WSDL file for web service.
how to create a WSDL file from a web service in asp.net