I have heard about that they are somewhat related to XML (not sure though). I heard that a web serivce resides on the server-side with a certain function. When the client (webpage) calls the function, it returns a certain result to the client. That's it? Can't you just do this with simple PHP, ASP, or any other server-side programming language? If not then really what is a web service and what is the main purpose of it?