Here are the basics:
- The new AJAX Framework
- AJAX forms, with a complete how-to and sample, easy to understand code.
- The fantastic new #states feature for dynamic forms.
We've made great strides in Drupal 7. Come and hear about it.
If you used AHAH in D6, you're probably afraid of AJAX forms. But if you never used #ahah, you'll walk out of the room feeling competent to try out #ajax. You can't believe how easy it is now.