Selenium IDE or Selenium on Rails?? Is there a BIG difference?

Hello again.

What I don't finish to understand is what is the big difference
between cases/suites that I can create and run in Selenium IDE and
cases/suites that i can create and run using Selenium on Rails.

Seeing Railscasts Episode 116 (http://railscasts.com/episodes/116-
selenium) they say the if you want to create "something more real
specific" you will move to Selenium on Rails (which doesn't work with
RoR 2.1 so far???).

But what are those "real specific" situations?
More AJAX?

Thanks for your answers.

I think http://github.com/ryanb/selenium-on-rails/tree/master actually
does work in Rails 2.1