Ajax isn’t a technology. It’s really several technologies, each flourishing in its own right, coming together in powerful new ways. Ajax incorporates:
1. Standards-based presentation using XHTML and CSS.
2. Dynamic display and interaction using the Document Object Model.
3. Data interchange and manipulation using XML and XSLT.
4. Asynchronous data retrieval using XMLHttpRequest.
In a nutshell, it is a method to interact with the server and send data back and forth without having to refresh the whole page, which makes applications faster and saves lots of internet bandwidth.
Here is how Ajax works in comparison to classical web application.