AJAX is an answer to a problem faced with standard web pages.
What is that problem? It's where actions that you perform on a web page cause the entire page to unload, and a new page to be loaded. When you click a link you are taken to a new page. When you submit a form you are taken to either a new page, or a completely new version of the same page.
The problem is that the existing page contents cannot remain, while information is communicated in the background, or content is transferred so that you can perform an update to only a small part of the page.
Aysynchronous means you can send data or make requests from the server without needing to reload the entire page
And that data is commonly transferred as JSON or
It is the technique of using that asynchronous communication which makes up fundamental core of what AJAX is.
A resource that has proven to be very useful when writing your own AJAX code is the [Bulletproof Ajax book, which also provides [url="http://bulletproofajax.com/code/"]code and working examples](http://bulletproofajax.com/).