The most common web services approach right now is SOAP over HTTP, where request and reply messages formatted in the SOAP schema for XML are passed between computers using the HTTP transport. The overall specifications for how this web services technology works is defined as the WS-I Basic Profile, which defines more clearly how the World Wide Web Consortium (W3C) specs go together.
Java has web services APIs specifically for supporting SOAP processing: JAX-RPC support using SOAP RPC to invoke Java methods. SAAJ supports sending XML documents over the web (document-style SOAP).
Some places to learn about SOAP:
- SOAP Tutorial (W3Schools)
- SoapRPC Tutorials (SoapRPC.com)
- Learn SOAP Tutorial (TopXML)
- Java and XML: SOAP (O'Reilly Media)