I was looking at our Gemfile at work today and noticed that we were not using the :assets group that is in the default Gemfile. When you create a new Rails application, that group will look like this:

group :assets do
 gem 'sass-rails', '~> 3.2.3'
 gem 'coffee-rails', '~> 3.2.1'
 gem 'uglifier', '>= 1.0.3'
end

A quick search helped me understand the purpose of this :assets group: it basically acts as a shortcut for other groups based on what you set in your config/application.rb file:

if defined?(Bundler)
 # If you precompile assets before deploying to production, use this line
 # Bundler.require(*Rails.groups(:assets => %w(development test staging)))
 # If you want your assets lazily compiled in production, use this line
 Bundler.require(:default, :assets, Rails.env)
end

Because I didn’t need the convenience of the assets group, I can comment out line 3 and uncomment line 5. Then I move the gems out of the assets group and delete that group. It helps me keep my Gemfile clean and allows me to organize it more freely. I usually group gems that do not belong to a specific group based on their role: # carrierwave, # front end, # authentication, etc. For example:

# mongo
gem 'bson_ext', '1.6.1'
gem 'mongo', '>= 1.4.0'
gem 'mongoid', '2.4.7'

Source: http://stackoverflow.com/questions/7351607/how-is-the-assets-group-in-rails-3-1-handled-by-bundler