A callback is commonly used in asynchronous programming, so you could create a method which handles the response from a web service. When you call the web service, you could pass the method to it so that when the web service responds, it call's the method you told it ... it "calls back".
In Java this can commonly be done through implementing an interface and passing an object (or an anonymous inner class) that implements it. You find this often with transactions and threading - such as the Futures API.
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/Future.html