What Sets Ajax Apart - Difference between Ajax and other technologies

While CSS, DOM, aynchronous requests and Javascript are all necessary components of Ajax, it is quite possible to use all of them without doing Ajax.
In a classic web application when compared to its Ajax counterpart, the user workflow is defined by code on the server and the user moves from one page to another, punctuated by the reloading of the entire page. During these reloads, the user cannot continue with his work. In an Ajax application, the workflow is at least partly defined by the client application, and contact is made with the server in the background while the user gets on with his work.
In between these extremes are many shades of gray. A web application may deliver a series of discrete pages following the classic approach, in which each page cleverly uses CSS, DOM, JavaScript and asynchronous request objects to smooth out the user's interaction with the page, followed by an abrupt halt in productivity while the next page loads. A JavaScript application may present the user with page-like pop-up windows that behave like classic web-pages at certain points in the flow. The web browser is a flexible and forgiving environment and Ajax and non-Ajax functionality can be intermingled in the same application.
What sets Ajax apart is not the technologies that it employs but the interaction model that it enables through the use of those technologies. The web-based interaction model to which we are accustomed is not suited to sovereign applications and new possibilities begin to emerge as we break away from that interaction model.
There are atleast two levels at which Ajax can be used - and several positions between these as we let go of the classic page-based approach. The simplest strategy is to develop Ajax-based widgets that are largely self-contained and that can be added to a web page with a few imports and script statements. Stock tickers, interactive calendars, and chat windows might be typical of this sort of widget. Islands of application-like functionality are embedded into a document-like web page. Most of the Google's current forays into Ajax fit this model. The drop-down box of Google Suggest and the map widget in Google Maps are both interactive elements embedded into a page.
If we want to adopt Ajax more adventurously, we can turn this model inside out, developing a host application in which application-like and document-like fragments can reside. This approach is more analogous to a desktop application or even a window manager or desktop environment. Google's gmail fits this model, with individual messages rendering as documents within an interactive, application-like superstructure.
In some ways, learning the technologies is the easy part. The interesting challenge in developing with Ajax is in learning how to use them together. We are accustomed to thinking of web applications as storyboards, and we shunt the user from one page to another following a pre-determined script. With the application-like functionality in our web application, we can provide the user with a more fine-grained handle on the business domain, which can enable a more freee-form problem solving approach to his work.