Depends on your definition of web service - typically, if you say Web Service, you're talking about a standardized piece of software that makes itself available of the internet, usually communicating by xml messaging. If you're working with these kinds of web services, you're likely developing an API - the very embodiment of xml message interfacing - I give you data, I want a response, I wait for the response, you give me the response, then you might send some data in return - this would happen in a split millisecond - if you build it right! 🙂