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:

require 'active_support/concern'

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:

require 'active_support/concern'

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
Methods
A
E
I
Class Public methods
extended(base)

Source: show | on GitHub 

# File activesupport/lib/active_support/concern.rb, line 99
def self.extended(base)
  base.instance_variable_set("@_dependencies", [])
end
Instance Public methods
append_features(base)

Source: show | on GitHub 

# File activesupport/lib/active_support/concern.rb, line 103
def append_features(base)
  if base.instance_variable_defined?("@_dependencies")
    base.instance_variable_get("@_dependencies") << self
    return false
  else
    return false if base < self
    @_dependencies.each { |dep| base.send(:include, dep) }
    super
    base.extend const_get("ClassMethods") if const_defined?("ClassMethods")
    if const_defined?("InstanceMethods")
      base.send :include, const_get("InstanceMethods")
      ActiveSupport::Deprecation.warn "The InstanceMethods module inside ActiveSupport::Concern will be "              "no longer included automatically. Please define instance methods directly in #{base} instead.", caller
    end
    base.class_eval(&@_included_block) if instance_variable_defined?("@_included_block")
  end
end
included(base = nil, &block)

Source: show | on GitHub 

# File activesupport/lib/active_support/concern.rb, line 121
def included(base = nil, &block)
  if base.nil?
    @_included_block = block
  else
    super
  end
end