Let us begin by defining RIA Services.
Formal definition (http://bit.ly/nmrOA9):
Microsoft WCF RIA Services simplifies the traditional n-tier application pattern by bringing together the ASP.NET and Silverlight platforms. RIA Services provides a pattern to write application logic that runs on the mid-tier and controls access to data for queries, changes and custom operations. It also provides end-to-end support for common tasks such as data validation, authentication and roles by integrating with Silverlight components on the client and ASP.NET on the mid-tier.
Now, we will define it more informally, according to our experience. The most usual case is that we have a database, probably running under SQL Server, and we create ...