Ajax stands for "Asynchronous Javascript and XML".
What AJAX seems to be is a way to use javascript without actually learning or writing javascript, or at least not very much javascript.
Browser dependent javascript idiosyncracies are transparently handled by a good ajax package.
Ajax is a way of getting interaction into web pages without continual page refreshes. This is my view of things as I lauch into learning this new technology.
Here are some links:
Ajax notes / tom@mmto.org