The interface of interest (a plain-old interface) is registered with Java Services. A provider (a Java class to provide that service, i.e. to implement that interface) is also registered with Java Services.
A client of Java Services simply asks for the interface, from a given server address and port. The client can then use the interface just like any other Java interface. However, any methods called on that interface, will go over the web to the server, and will actually be provided by an instance of the registered provider.
Java Services also allows callbacks from the provider back to the client.
To see a detailed technical example, please visit the How-To page.