I find a lot of this AJAX stuff a bit of a hype. Lots of people have been using similar things long before it became “AJAX”. And it really isn’t as complicated as a lot of people make it out to be. Here is a simple example from one of my apps.
Normally I don’t bother too much with these “30 second tutorials” you see posted all the time, but this one is great. If you know even a little PHP (or some other server side scripting languages) you should be able to follow this tutorial.