NET RIA Services is a set of tools and libraries that make developing data-centric RIA’s (Rich Internet Applications) much, much easier.

Basically if you are writing a Silverlight application that will ever need to talk to your back-end servers, then .NET RIA Services will prove to be very useful.

Check-out this post by Bruno Terkaly for a great walk-thru:
http://blogs.msdn.com/brunoterkaly/archive/2010/01/28/ria-services-displaying-data-with-silverlight-4-beta-visual-studio-2010-beta-2.aspx 

Where he covers the following topics on RIA Services:

• How to work with databases and how to incorporate an ADO.NET Entity Model to our RIA Services application.
• How the relationship works between the “Domain Service” on the server and the “Domain Context” on the client (Silverlight app) to supply “entities” to the browser application (Silverlight).
• How to add custom views to the Silverlight client. This a critical skill moving forward as we expose more and more data to the end user.