AJAX is a set of interactive and interrelated technologies that makes multitasking easier than conventional technologies. AJAX technologies include -
JavaScript
The text-based programming language of the web, JavaScript is the glue that brings together all other elements in AJAX. When an event occurs in a website, a JavaScript function is executed.
HTML/XHTML and CSS
These technologies are responsible for the presentation of the web content such as its style, font, etc.
DOM
DOM is an API that displays web pages and interacts with data.
XMLHttpRequest
It is used to establish asynchronous communication with the server.
XML or JSON
XML is used for communicating to and from the server. It is being replaced by JSON or Javascript Object Notation these days. JSON is lightweight and faster than XML.