A web service is a way of communicating or interacting machine-to-machine over a network using Internet protocol to exchange data. HTTP and XML is the basis of web services. The software system that requests data is a service requester, whereas the software system that would process the request and provide the data is a service provider.
The rules for communication describing web services and how to access them are defined in a file called WSDL, stands for Web Services Description Language. WSDL is written in XML. WSDL is a document written in XML. The document describes a Web service. It specifies the location of the service and the operations the service exposes.
A directory called UDDI (Universal Description, Discovery and Integration) defines which software system should be contacted for which type of data. Once the software system finds out which other system it should contact, it would then contact that system using a special protocol called SOAP (Simple Object Access Protocol). The service provider system would first validate the data request by referring to the WSDL file, and then process the request and send the data under the SOAP protocol.