In Traditional web applications, you request a page and the the server, process the request, makes some database calls if needed and put your data and HTML together and pass on to the client.This model has now changed with RIA, using AJAX, Silverlight and others Take for an example a hotel reservation page that uses AJAX for user interactions. Maybe the intital page loads with all available countries in a combo box. and when the user selects a country, all the hotels for that country is retrived through a AJAX call. Now, this is different form the typical web model, initially by requesting this page, the server send out HTML and presentation logic (javascript in this case for AJAX) to the client. The request for the hotels for a particular country is a pure data call, where only data flows in the form of XML. This example was just used to demonstrate the change from the traditionl web model, but you would find a lot more pure data calls then just one (like in this case) in real world w...