module.rb

module A
  def some_instance_method
    "hi"
  end

  module ClassMethods
    def foo
      "asdf"
    end
  end
end 

usage.rb

class Blah
  include A
  extend A::ClassMethods
end

$ Blah.new.some_instance_method
# => "hi"

$ Blah.foo
# => "asdf"