Marionette.js is a JavaScript framework that builds upon Backbone.js to provide additional structure and functionality. It fills gaps in Backbone by implementing common patterns for views like ItemView, CollectionView, CompositeView, and LayoutView. These views provide functionality for rendering individual models, collections of models, composite views that combine a collection within a template, and complex nested layouts with multiple regions respectively. The framework is actively maintained by a core team and large community and used by many large applications for its flexibility, separation of concerns, and ability to build complex UIs through nesting of views and regions.