Selenium is a suite of tools to automate web app testing across many platforms. Cucumber is a BDD framework that allows you to write things in English (or whatever language you want, really), and have that execute as code. Put those together with webrat and rspec and you have a pretty mean stack to test your ruby on rails web application with. Sort of.

I've been having some problems with it, getting it set up, but it's coming along.

This isn't a post about how to setup Selenium from scratch. There are other resources out there for that, like this page

I'm just going to rant about some of the problems I've come across and some fixes for them.

1. All users have wrong passwords, no data in DB. You can't have transactional fixtures when using selenium, so turn those off in your env.rb file. You can use database_cleaner to try and help with that, but I'm still working out kinks with that too it seems. A step in the right direction.
2. current_url checks fail randomly. Sometimes selenium doesn't wait long enough for pages to load, especially after form POST's. You need to tell selenium to wait after certain things by calling selenium.wait_for_page_to_load This of course only works when using selenium so wrap it up to only call this if you are using selenium, so that you can still run your tests in normal rails/webrat mode and not die in a fire.
3. Database busy/timeout when using sqlite. This came up a few times randomly, but adding timeout: 1000 to the test database in database.yml worked wonders. Some say it was because of the transactional fixtures problem, but it occurred with me even when they were turned off.
4. Catch those confirms. You have to catch confirms in selenium, with selenium.get_confirmation, so wrap that up, and put it in your step definitions after things that you know pop up confirmation dialogs.

The selenium-client docs also provide some good information if you are having problems. Get them here