The first question is how the client can convey its intentions to the server. How does the server know a certain request is a request to retrieve some data, instead of a request to delete that same data or to overwrite it with different data? Why should the server do this instead of doing that?
I call the information about what to do with the data the method information. One way to convey method information in a web service is to put it in the HTTP method. Since this is how RESTful web services do it, I’ll have a lot more to say about this later. For now, note that the five most common HTTP methods are GET, HEAD, PUT, DELETE, and POST. This is enough to distinguish between “retrieve some data” (GET), “delete that same data” (DELETE), and “overwrite it with different data” (PUT).
The great advantage of HTTP method names is that they’re standardized. Of course, the space of HTTP method names is much more limited than the space of method names in a programming language. Some web services prefer to look for application-specific method names elsewhere in the HTTP request: usually in the URI path or the request document.