What is Ajax

Ajax (also AJAX; an acronym for asynchronous JavaScript and XML) is a group of interrelated web development methods used on the client-side to create interactive web applications. With Ajax, web applications can send data to, and retrieve data from, a server asynchronously (in the background) without interfering with the display and behavior of the existing page. Data are usually retrieved using the XMLHttpRequest object. Despite the name, the use of XML is not needed (JSON is often used instead), and the requests need not be asynchronous.

I often use Ajax in my HTML5 based mobile applications to provide enhanced, richer functionality which would not be possible with CSS and HTML alone.