AJAX refers to the technique of asynchronous (as necessary) loading of data. These data can be pieces for program execution or delayed loaded components of the user interface. The asynchronous loading is caused by the Javascript code, running when you view a Web page in the browser. The aim of this approach is to offer the user a desktop-like application in the web browser without unreasonable increase of Data transfer rates and load times.
Prerequisite for the use of AJAX is that the browser has JavaScript enabled.