How AJAX works?
Ajax works on xmlHttp Request to server and Client.

1. Client Browser (Sends Request)
2 Server (Process the Request and sends the Response)
3. Client Browser ( process the request using Javascript and Displays the Content)

1. Clients creates xmlHttpRequest to server.
2. Server process the HttpRequest from client. And send response to the client browser.
3. Client browser process the returned data using javascript. Content of the page will be updated.

Do you know 95% of AJAX is JAVASCRIPT only.
With AJAX we can do lot of things display a result dynamically as well as differently in appearance wise.  Thing of google worlds top search engine uses AJAX as a main source. Did you Noticed while typing in a search box in google a drop down search suggestion will open. Have you wondered about how it works? Its all of AJAX.
You can load the content dynamically as you want to display it in time. For Example If you having a very big content in your Database or some thing which is Large to load in browser at that time you can show a loading image so user can easily identify that content is loading or processing. After content has been been loaded it will be displayed to user browser.
With AJAX we can fetch the data from database and parse that as XML and display it in browser as you like. AJAX is very faster in loading its a great thing to use AJAX.
However it is I wrote many examples of AJAX coding in my tutorial.
So learn AJAX and play with it.