The traditional web model is based on a series of successive queries and responses (the sequential navigation from one web page to another web page). AJAX allows you to change only part of the web page, which needs to be updated by creating a local HTTP query and modifying all or part of the site, based on HTTP request recovered.
The AJAX is neither a technology nor a programming language, but refers to a method for developing web pages using the following technologies:
HTML and CSS for presentation
DOM (Document Object Model) to represent objects in the web page