Conventional web applications transmit information to and from the sever using synchronous requests. It means you fill out a form, hit submit, and get directed to a new page with new information from the server.
XML is commonly used as the format for receiving server data, although any format, including plain text, can be used.
AJAX is a web browser technology independent of web server software.
A user can continue to use the application while the client program requests information from the server in the background.
Intuitive and natural user interaction. Clicking is not required, mouse movement is a sufficient event trigger.
Data-driven as opposed to page-driven.