class_methods.rb

class Post < ActiveRecord::Base
  # has_many is inherited from AR::Base
  has_many :comments
end

class PostsController < ActiveController::Base
  # before_filter is inherited from ActionController::Base
  before_filter :authenticate

  def authenticate
    raise Unauthenticated unless user.logged_in?
  end
end

# this allows frameworks/libraries to build features that appear built-in

class Person
  # attr_accessor is defines getters/setters, but is actually inherited from Class
  attr_accessor :name, :address
end