Gem、Railtieプラグイン、Engine（full/mountable）の違いとそれぞれの基礎情報 ref: http://qiita.com/kidachi_/items/565c2c077ae8d15fe3a8

application.rb

# Pick the frameworks you want:
require "active_model/railtie"
require "active_record/railtie"
require "action_controller/railtie"
require "action_mailer/railtie"
require "action_view/railtie"
require "sprockets/railtie"

file0.txt

bundle gem <gem_name> -t

file1.txt

module MyPlugin
  class Railtie < Rails::Railtie
    # config.my_plugin = ...
    # initializer "my_lugin.set_configs" ...
  end
end

file12.txt

 $ rake routes
     Prefix Verb URI Pattern    Controller#Action
my_engine      /my_engine   MyEngine::Engine

Routes for MyEngine::Engine:
   foo GET  /foo(.:format) my_engine/bar#foo

file3.rb

module Rails

  class Railtie
  end

  class Engine < Railtie
  end

  class Application < Engine
  end

end

file4.txt

rails plugin new <plugin_name>

file5.txt

rails plugin new <engine_name> --full

file6.txt

rails plugin new <engine_name> --mountable

file9.txt

 $ rake routes
Prefix Verb URI Pattern    Controller#Action
   foo GET  /foo(.:format) bar#foo

routes.rb

MyEngine::Engine.routes.draw do
  get 'foo' => 'bar'
end