A fast and feature-rich Drupal distribution, Lightning is a base profile that can be used as a whole or in parts by other projects.
One example of this is Acquia's Demo Framework - a robust suite of demo scenarios that are built on top of the Lightning components.
The intent of this project is to provide a lightweight framework for building working solutions in Drupal.
Our components are managed as decoupled Lightning Features.
If you are interested in building a site of your own "on top of Lightning" you'll want to include Lightning Features as a part of your install profile. This particular project (Lightning) showcases all the components working together.