(Rails) What is “RJS”?

This Railscast gives a nice example of using RJS to add and remove form fields dynamically without hitting the server with an ajax call.

RJS is a template (similar to an html. Erb file) that generates JavaScript which is executed in an eval block by the browser in response to an AJAX request. It is sometimes used (incorrectly?) to describe the JavaScript, Prototype, and Scriptaculous Helpers provided by Rails.