Posting a message over http is what you already know .
Webservice is, createing a message in a specific protocol(SOAP envelope which defines an xml format) and then post
it over the http . On the serverside the soap envelope recieved is decoded and then call the corresponding service (maybe
anything not only java) and then again send the response in the xml format to the client .
The advantage is since you are using a predefined xml format , you can communicate between different platforms i.e. a java
program can communicate to a c# program whithout even needing to know that it is talking to a c# program.