Behat is a tool that makes behavior driven development (BDD) possible. With BDD, you write human-readable stories that describe the behavior of your Drupal site. These stories can then be auto-tested against your website, whether in the midst of development, or on a live site. And yes, it’s as cool as it sounds!
Behat, if embraced by enough Drupal folks, has the potential to vastly improve the way we build and test Drupal websites.