Rails doesn't have it's own JS library. It initially used only the prototype library (which exists outside of Rails), but with Rails 3 any javascript supplied by Rails was made library agnostic.

RJS is a rails specific thing, but it's more a method of generating javascript inside of rails than a JS library in it's own right.

This change simply changes the default from Prototype to JQuery - a long-overdue change IMO, since the vast majority of the Rails community has been using solely JQuery for quite a long while now.

Here's hoping that the same treatment is done to RSpec vs. TestUnit.