Ajax

Ajax (Asynchronous JavaScript + XML) is a collection of  Web development techniques used on the client-side to create asynchronous Web applications. Using  Ajax, a Web applications can send data to, and retrieve data from, a server asynchronously (in the background) without interfering with the display and behavior of the existing page. Data can be retrieved using the XMLHttpRequest object.

Ajax is not a single technology, but a group of technologies. HTML and CSS can be used in combination to mark up and style information. The DOM is accessed with JavaScript to dynamically display – and allow the user to interact with – the information presented. JavaScript and the XMLHttpRequest object provide a method for exchanging data asynchronously between browser and server to avoid full page reloads.

Examples

Sample code for implementing - check availability option in registration form - using Ajax, PHP and MySQL