Example: a reservations system. In the RPC-style case, an IDL or WSDL might declare a method "NewReservation", and it would tell you some parameters: NewReservation (name, flight number, airline, date); you write client programs that collect that information from a user, and invoke the RPC.
An HTTP/HTML reservation system, however, constructs a form with input elements named "name", "flightnumber", "airline", and "date"; the user fills in the form and pressing the Submit buttons sends the information to the service. So the client program, a web browser, never knew the semantics of the service. The human operator did, of course: he read the descriptive text in the form and put his name in the proper box.
Now, that's fine for human operators. But what about machine to machine operations? In the RPC case, the client program is essentially the same as in the human case, only now the program collects the parameters from a database rather than from a live user. The client program, as before, is compiled against the IDL or WSDL. In the RPC case, we presume we can just invoke the service "out of the blue." We needn't have obtained any information from the service before hand; we just connect and invoke.
In a REST-style service, we want an analogy to the self describing hypermedia we have in the HTML scenario. First let's assume we'll use some form of XML as our hypermedia. It's easy to imagine a
Could you have done all of this using an RPC-style architecture? Maybe. You could have retrieved WSDL from some well known service. You would do that each time you want to invoke the service, to emulate the self-describing part. Then you could dynamically construct the RPC call -- the serialization bit wouldn't be hard -- if you also had a mapping of the RPC parameters to your data model. How would you do that? There would need to be some semantic description of the service parameters and you would need a mapping of that description to your own data model. Could you use some RDF to describe these semantics? Probably. But it's not a system designed from the ground up to be self describing.