AJAX

Ajax is a group of inter-related web development techniques used for creating interactive web applications. It stands for Asynchronous JavaScript and XML and is the basis on which many of the Web 2.0 applications are formed.

A primary characteristic is the increased responsiveness and interactivity of web pages achieved by exchanging small amounts of data with the server "behind the scenes" so that entire web pages do not have to be reloaded each time there is a need to fetch data from the server. This is intended to increase the web page's interactivity, speed, functionality, and usability.

Wikipedia

Long story short, AJAX is a combination of XML and JavaScript web coding languages that are used to build websites. They are not visible languages like HTML is. Instead they work behind the scenes to load data asynchronously. This then means that their use ensures that the website loads faster than if it were written entirely in HTML because it only loads certain parts in the background. AJAX also allows for things that are beyond the boundaries that encompass HTML, often resulting in a more interactive web experience for the user.

An example of this sort of boundary pushing is perhaps more obvious that it would at first seem. Ever been looking for an item of some sort on a website and found that when you click on the image, the image loads with out taking you to a new page? Yes? You've found yourself an example of AJAX! Basically the AJAX coding sends request data to the server on which everything loads from, and the server sends back that which was requested. So in this case you click on the button wanting to see the image so the coding sends a request for the image to be displayed to the server. The server then loads and displays the image on the webpage with out having to reload it or directing you to another place.

For an example of how to use AJAX head over here! And for an example of an AJAX website head over here!