A typical module looks like this:

module M
  def self.included(base)
    base.extend ClassMethods
    scope :disabled, where(:disabled => true)
  end

  module ClassMethods
    ...
  end
end

By using ActiveSupport::Concern the above module could instead be written as:

module M
  extend ActiveSupport::Concern

  included do
    scope :disabled, where(:disabled => true)
  end

  module ClassMethods
    ...
  end
end

Moreover, it gracefully handles module dependencies. Given a +Foo+ module and a +Bar+ module which depends on the former, we would typically write the following:

module Foo
  def self.included(base)
    base.class_eval do
      def self.method_injected_by_foo
        ...
      end
    end
  end
end

module Bar
  def self.included(base)
    base.method_injected_by_foo
  end
end

class Host
  include Foo # We need to include this dependency for Bar
  include Bar # Bar is the module that Host really needs
end

But why should +Host+ care about +Bar+'s dependencies, namely +Foo+? We could try to hide these from +Host+ directly including +Foo+ in +Bar+:

module Bar
  include Foo
  def self.included(base)
    base.method_injected_by_foo
  end
end

class Host
  include Bar
end

Unfortunately this won't work, since when Foo is included, its base is the Bar module, not the Host class. With ActiveSupport::Concern, module dependencies are properly resolved:

module Foo
  extend ActiveSupport::Concern
  included do
    class_eval do
      def self.method_injected_by_foo
        ...
      end
    end
  end
end

module Bar
  extend ActiveSupport::Concern
  include Foo

  included do
    self.method_injected_by_foo
  end
end

class Host
  include Bar # works, Bar takes care now of its dependencies
end