Module - Include or Extend? and Mixin: http://surya.rajtripathi.com/2013/12/module-include-or-extend-and-mixin.html

include_vs_extend.rb

module InstanceModule
  def track
    "Tracking todo: #{self.name}" 
  end
end

module ClassModule

  def class_name
    "Tracking class: #{self.name}"
  end
end

class Todo
  include InstanceModule
  extend ClassModule

  attr_reader :name

  def initialize(task_name)
    @name = task_name
  end
end

t = Todo.new('read a blog')
puts t.track
puts Todo.class_name