Yep, AJAX is not Magic, its Voodoo...
Plenty of example are on offer in the threads if you use the search tool.
A prime example of AJAX in action is sites that check if a username is in use when your signing up to them, this then updates the web page with data or returns an OK depending on what is found.
You can submit complete forms via AJAX as long as the recieving script is scripted to cope with the data being pushed.
@Mindzai - whilst that is true, you are however limited by two other factors, the POST limit as set by PHP config on web hosts and the database, if the post limit exceeds the field length then the data is truncated or returns an error depending on how graceful the set up is.