What is "web-service"
A Web Service is a method of communication between two electronic devices through the World Wide Web. A web service is a software function provided in a network address through the web or the cloud, it is a service that is Always on, as in the concept of computation available.
W3C defines a Web service as:
[ ...] A software system designed to support machine-to-machine interoperable interaction on a network. It has an interface described in a machine-readable format (specifically wsdl). Other systems interact with the web service in a manner prescribed by its description using messages Soap, normally transmitted over HTTP with an XML serialization in conjunction with other web-related standards.