Depending on your education or where you live AJAX can mean one of at least four things, if you studied classics you will known Ajax from Homer's Iliad, if your in to Football then you might know Ajax of Amsterdam and if you live in the UK and are over 30 you will know Ajax toilet cleaner. For me fitting in all three categories it summons up visions of a football playing Greek hell bent on cleaning the toilets after the match.
This means web sites can behave much more like client server apps, so that list can populated when another value on the page changes, validation can take place almost instantly and so on.
I'm building a prototype of an exisitng web site we run, for this I'm using PHP and MySQL, this seems a great choice but the thing with AJAX is that the language is unimportant. One thing I have found however is that while there is plenty of buzz around on the web about AJAX, examples seem to be hard to come by, it may be that there are plenty of examples around but that it's swamped by all the blog and magazine articles giving us just a taste. I've managed however to knock something up, it needs a bit of work but it's looking good so far.
Anyway I'll keep you posted and if it works out I'll provide a URL of the prototype.