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 (#116 Selenium - RailsCasts 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 actually does work in Rails 2.1