From http://www.w3schools.com (Copyright Refsnes Data)

You Have Learned Web Services, Now What?

Web Services Summary

This tutorial has taught you how to convert your applications into web-applications.

You have learned how to use XML to send messages between applications.

You have also learned how to export a function (create a web service) from your application.

The next step is to learn about WSDL and SOAP.

WSDL

WSDL is an XML-based language for describing Web services and how to access them.

WSDL describes a web service, along with the message format and protocol details for the web service.

If you want to learn more about WSDL, please visit our WSDL tutorial.

SOAP

SOAP is a simple XML-based protocol that allows applications to exchange information over HTTP.

Or more simply: SOAP is a protocol for accessing a web service.

If you want to learn more about SOAP, please visit our SOAP tutorial.