AJAX (Asynchronous Javascript And XML) is a popular technique for creating interactive websites, though other technologies (e.g. JSON) are often used instead of XML.

AJAX is a set of techniques that run on the client browser to create interactive websites. It allows updates to the current page to be made in more or less real-time without having to reload the page from the server. AJAX has been a substantial technological advancement driving the development of so-called "Web 2.0" applications or RIA (Rich Internet Applications) by facilitating a more seamless refresh and partial refresh of internet application interfaces.

AJAX on Wikipedia