I have looked at the definition of a Web Service on wikipedia and other online sources. They all describe using WSDL, XML schema, and SOAP. The first web service project I worked on used all of these, plus Axis 2 to make the SOAP interfaces more straightforward.
For our most recent project we needed something simpler, and happened onto this approach. The available descriptions of web services don't say much about it, unless it's related to REST.