AJAX:
Defined:
AJAX = Asynchronous JavaScript and XML.
What is does:
AJAX is the process of exchanging data with a server, updating part(s) of a web page, without reloading the whole page.
- WC3 - AJAX Tutorial
A good place to start learning about AJAX
- jQuery API - AJAX
If you like jQuery you will love its AJAX capabilities!
- jQuery + AJAX tutorials
Various tutorials in reference to jQuery and AJAX can be found here.