This is a good summary for those that have not heard the term yet:
http://en.wikipedia.org/wiki/AJAX

maps.google.com is a great example of an AJAX app; not that I will be doing this kind of development anytime soon ever, but it is useful to know of new trends. Personally I would choose SmartClient for internal apps and some kind of Flash based solution for complex public apps... there are some interesting happenings in the .Net + Flash space. Again not my cup of tea, but worth being aware of.