Starting from static web pages (.HTML, .HTM, etc) to dynamism with server scripts (.asp, .jsp, etc) to OO enabled server scripts (ASP.NET) to powerful client applications (ASP.NET + AJAX), THE web has already stepped into rich model of first class web applications delivery to end users.
Making the client rich alone will not work. And that is where .NET RIA services comes into play. There is a nice introductory page released recently, which will clear all your questions and give very good understanding of overall picture.
Getting started with the .NET RIA Services