A Web service is a software system identified by a URI whose public interfaces and bindings are defined and described using xml. Its definition can be discovered by other software systems. These systems may then interact with internet protocols.
(OR)
A Web service is a software application, accessible on the web through URL, that is accessed by clients using XML based protocols, such as SOAP(Simple Object access protocol) sent over accepted Internet protocols such as HTTP. Client access a web service application through ts interfaces and bindings, which are defined using XML artifacts sch as a WSDL (Web Service Definition Language) file.