The core justification for Ajax style programming is to overcome the page loading requirements of HTML/HTTP-mediated web pages. Ajax creates the necessary initial conditions for the evolution of complex, intuitive, dynamic, data-centric user interfaces in web pages-the realization of that goal is still a work in progress.
Web pages, unlike native applications, are loosely coupled, meaning that the data they display are not tightly bound to data sources and must be first marshalled (set out in proper order) into an HTML page format before they can be presented to a user agent on the client machine. For this reason, web pages have to be re-loaded each time a user needs to view different datasets. By using the XMLHttpRequest object to request and return data without a re-load, a programmer by-passes this requirement and makes the loosely coupled web page behave much like a tightly coupled application, but with a more variable lag time for the data to pass through a longer "wire" to the remote web browser.
Source: Wikipedia.org
