AJAX stands for Asynchronous JavaScript and XML. Any server
side technology that supports JavaScript also supports AJAX. AJAX is a browser
technology, and is therefore independent of web server platforms. AJAX is not a programming language, but a technique for creating better, faster, and more
interactive web applications.
If we are using PHP or any server side technology and need
to extract data from storage on a server (e.g. a database or a file), we will
have to make an HTTP request (either POST or GET) to get the data. Once the
data is received the web page will need to be reloaded to show the data. Using AJAX technology we can request and receive the data from the server in background and then
display it on the page without a reload. AJAX uses HTTP requests for this. With
AJAX, JavaScript communicates directly with the server, through the
JavaScript XMLHttpRequest object (XML over HTTP). With an HTTP request, a web
page can make a request to, and get a response from, a web server without
reloading the page.