The major change in Rails 3.1 is the Assets Pipeline. It makes CSS and JavaScript first-class code citizens and enables proper organization, including use in plugins and engines.
The assets pipeline is powered by [Sprockets](https://github.com/sstephenson/sprockets) and is covered in the [Asset Pipeline](asset_pipeline.html) guide.
The assets pipeline is powered by [Sprockets](https://github.com/rails/sprockets) and is covered in the [Asset Pipeline](asset_pipeline.html) guide.