Web applications have entered a new era driven by web site goals such as fast response to user actions and user collaboration in creating and sharing web site content. The popular term attributed to these highly responsive and often collaborative sites is Web 2.0. Some prime examples of Web 2.0 are web sites such as Google Maps and Flickr. Google Maps offers a highly responsive user interface (UI). For instance, you can view a map, then move your cursor across it to see adjacent areas almost immediately. Flickr is a site on which users store and share photographs — users manage almost all the site’s content.

Other Web 2.0 sites provide a similarly rich user experience by doing things such as integrating services from other web sites or incorporating a steady stream of new information. For example, the Google map service can be brought into another web site, such as a site for purchasing cars, to present a map that highlights the location of auto dealerships that sell a particular car model. The term used for these site integrations is “mashups.” Or a sports-oriented site can continually update scores without requiring the user to request a page update.

What is AJAX?
A number of excellent articles that describe AJAX are available, for example, Asynchronous JavaScript Technology and XML (AJAX) With Java 2 Platform, Enterprise Edition. In brief, AJAX is a set of technologies that together allows a web site to be — or appear to be — highly responsive. AJAX enables this because it supports asynchronous and partial refreshes of a web page.

A partial refresh means that when an interaction event fires — for example, a user enters information into a form on a web page and clicks a Submit button — the server processes the information and returns a limited response specific to the data it receives. Significantly, the server does not send back an entire page, as is the case for conventional, “click, wait, and refresh” web applications. Instead, the client then updates the page based on the response. Typically this means that only part of the page is updated. In other words, the web page is treated like a template: The client and the server exchange data, and the client updates parts of the template based on the data the client receives from the server. One way to think of this is that web applications that use AJAX are driven by events and data, whereas conventional web applications are driven by pages.

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