A Drupal app takes the functionality from a group of modules or features, wraps it up in a meaningful and extremely user-friendly fashion, and then makes sure it plays well with other apps. Since it can be powered by modules and features, a Drupal app includes all the right roles and permissions, as well as content types, taxonomies, views, settings and blocks, resulting in a one-click installation and an enhanced user experience.
The server-side of the consumer app runs on Heroku and is built with Drupal.