Excerpt from MSDN documentation:

A common problem when developing an n-tier RIA solution is coordinating application logic between the middle tier and the presentation tier. To create the best user experience, you want your RIA client to be aware of the application logic that resides on the server, but you do not want to develop and maintain the application logic on both the presentation tier and the middle tier. RIA Services solves this problem by providing framework components, tools, and services that make the application logic on the server available to the RIA client without having to manually duplicate that programming logic. You can create a RIA client that is aware of business rules and know that the client is automatically updated with latest middle tier logic every time that the solution is re-compiled.

The following illustration shows a simplified version of an n-tier application. RIA Services focuses on the box between the presentation tier and the data access layer to facilitate n-tier development with a RIA client.

WCF RIA Services n-tier application architecture

WCF RIA Services focus into the box between Presentation Logic and Data Access Layer providing Data Services on the server accessible to the client via Data Context.

There are many technologies that can be used to create the web application. The next picture illustrates this.

Various technologies can be used in Presentation tier and Data tier

…to be continued (this article is not complete yet)