A web service is exactly what it says; it's a service on the web. So what "service" do web services provide?
A web service's goal is to provide raw data in XML format to any application that makes a request to it. That may not make sense by itself, so here is exactly what a web service does.
A web service sits on a server much like any server-side page, and when a request is made to it, the web service will perform a desired task and return data in the form of XML. XML, as you learned in the previous chapter, is a language that nearly any application can read because it is in fact a meta-language made up of customized tags containing well-formed and structured data.
That is what a web service is and does, but that doesn't explain why anyone should use them.