AJAX is short-form of Asynchronous JavaScript and XML.
AJAX is a web development technique by using it you can create faster web applications with HTML, CSS and Java Script.
By using Ajax, web applications can send and retrieve data from a server asynchronously without reloading existing page.
The main purpose of AJAX is to get and set data dynamically without reload whole existing web page.
Many popular applications like Gmail, Youtube, Google Maps, Facebook uses Ajax technology.