Capybara is a web application testing framework for Rack applications, such as Rails, Sinatra and Merb.
Capybara aims to simplify the process of integration testing Rack applications, such as Rails, Sinatra or Merb. Capybara simulates how a real user would interact with a web application. It is agnostic about the driver running your tests and currently comes bundled with rack-test, Culerity, Celerity and Selenium support built in.
A complete reference is available at rubydoc.info.
A great capybara cheat sheet is available here.