I will not go into any kind of detail about what AJAX is, or how it works. I do, however, insist that before you continue with this article that you have a clear understanding of the technical workings of AJAX. By this, I mean that you must know what AJAX is in its purest form, and what it does and does not stand for. There is a big diffrence between fancy javascript and AJAX. For that understanding, I refer you here: Build Your Own AJAX Web Applications. This is a great article over at SitePoint written by Matthew Eernisse that covers the basics, as well as goes through creating a sample application. This should more than prepare you for this article.
The singleton design pattern is a way of enforcing only one instance of an object. This is achieved by making 3 fairly simple steps to a class. Firstly making the constructor private, creating a static member variable that will contain the instance and then creating a static method for accessing the instance.
Joining mailing list will entitle you
to receive occasional emails informing you of news and
updates to the site and any special offers that may be
of interest to you.