What is Ajax?

Short for Asynchronous JavaScript and XML, it is a term that describes a new approach to using a number of existing technologies together, including the following: HTML or XHTML, Cascading Style Sheets, JavaScript, the Document Object Model, XML, XSLT, and the XMLHttpRequest object.
When these technologies are combined in the Ajax model, Web applications are able to make quick, incremental updates to the user interface without reloading the entire browser page. [Source: mozilla developer center]