As describe earlier, ajax is not a technology but group of inter-related technologies. AJAX technologies includes:
HTML/XHTML and CSS
These technologies are used for displaying content and style. It is mainly used for presentation.
It is used for dynamic display and interaction with data.
XML or JSON
For asynchronous communication between client and server. For more visit next page.
It is used to bring above technologies together.
Independently, it is used mainly for client-side validation.