I'm tinkering a bit with Cucumber and Webrat for my day job and am very excited by some of the prospects for our QA group's automation efforts. Along the way I'm finding that I have to explain how all the moving parts relate to each other, so I made this diagram:
Cucumber, based on RSpec, uses Webrat to drive Selenium. Seems to make it all make sense to the folks with the questions.
(and thanks, brynary + all the webrat contributors, aslakhellesoy + all the cucumber contributors, and dchelimsky + all the rspec contributors, and the whole selenium crew ... this is very, very cool stuff).