I really can't find the right Google terms to track this down. But you're all quite smart so thought I'd post it here.

You know how sometimes you are on a good site and you click a button (like to submit a form) and the form doesn't go away, instead, the foreground becomes transparent and it contains a message of some kind, or another page. The message is at the forefront but the previous page is still mostly visible behind it - how do they do that? I'm assuming it's an AJAX-esque trick.