Lightning Dart is a library for developing web applications based on the CSS framework Salesforce Lightning Design System.
Written in Dart ("Dart - the better Javascript"), it compiles into Javascript, so the delivery is a "one page apps".
The Framework is not dependent on salesforce.com functionality and can be used independently.
The Salesforce Lightning CSS framework is similar to Bootstrap but uses more recent CSS technology and concentrates on building Web Applications, not Web Sites.