The classic web application model works like this: Most user actions in the interface trigger an HTTP request back to a web server. The server does some processing—retrieving data, crunching numbers, talking to various legacy systems—and then returns an HTML page to the client. It’s a model adapted from the Web’s original use as a hypertext medium, but as fans of The Elements of User Experience know, what makes the Web good for hypertext doesn’t necessarily make it good for software applications.

Web as hypertext 

vs

Web as app

http://www.adaptivepath.com/ideas/ajax-new-approach-web-applications