I saw Maurice de Beijer recently published a good introduction article on .NET RIA Services.
The post starts with a very good introduction:
RIA, short for Rich Internet Applications, is a bit of an umbrella term for all sorts of applications delivered through the browser. The key aspect however is that they deliver some business function and are not just about flashy graphics. Business applications tend to work with data and other business resources so they are usually built in the standard N tier architecture. If we take a look at this N tier architecture for the most common type of business resource, the database, we typically see the Create, Read, Update, Delete, (CRUD) pattern appear all the time. While implementing the CRUD pattern in Silverlight isn’t extremely difficult the very fact that the Silverlight application runs in the browser without direct database access and all server communication is done asynchronously makes this harder than it needs to be. This is exactly one of the problems the .NET RIA Services is trying to solve. Of course there is more to the .NET RIA Services and the standard CRUD operations is just one of the issues addressed. As we will see in this article it addresses much more by including things like data validation, general communication, keeping client and server code synchronized and more.