AJAX stands for Asynchronous Javascript
and XML. Many popular websites make use of AJAX, for example Google
and Twitter. To understand why it's being used, we have to
understand how the traditional World Wide Web was meant to be built.
The web was intended to work like this: you use a web browser
application (eg. Internet Explorer, Chrome, Firefox, Safari) on your
computer to visit a web page, by giving the browser that page's URL
address (eg. http://www.google.com). The browser, or client,
connects to a web server via the internet and requests that web page,
which is merely a text file, specifically an HTML file. That server
serves that file back to the client browser, and the browser displays
that page, also requesting any additional files such as images needed
to completely display that page. You can then click on a hyperlink
to visit another web page, and your browser goes and connects to
whatever web server is that file is stored on to get that page. But
as the web has matured, websites have gotten more complicated,
bigger, and more visual, and the basic HTML web model has become
obsolete. We moved from mostly text-based pages to websites
consisting of many web pages, all with consistent visual elements
(eg. logo in the top left, a navigational menu along the top or side,
a footer) which are there to make the web more user friendly. We've
also developed new technologies, like CSS, Javascript, and PHP, to
supplement our basic HTML websites, and one of these technologies is
AJAX.
