What's the Difference between the Rspec and Rspec-Rails Gem?

I have the following in my GEMFILE:

group :development do 
  gem 'rspec-rails'
  gem 'annotate-models'
end 

group :test do 
  gem 'rspec'
  gem 'webrat'
  gem 'spork'
  gem 'factory_girl_rails'
end 

What's the difference between 'rspec' and 'rspec-rails'?