It's a full stack framework. You can include JavaScript for your views, probably most commonly used to show, hide, and insert and remove elements from the DOM.
To expand on this, the tool of choice for the frontend part used to be PrototypeJS. In Rails 3.1 the relevant portions were decoupled and became options rather than baked-in, with jQuery as the default but still an option among several: http://weblog.rubyonrails.org/2011/4/21/jquery-new-default/