An example of instance vs class variables

dog.rb

class Dog

  # class variable
  @@default_description = "A dog is a furry mammal."

  attr_accessor :name
  attr_accessor :age
  attr_accessor :description

  def initialize(name, description)
    @name = name  # instance variable
    # Set the description
    #  @description = description
    if description
      @description = description
    else
      @description = @@default_description
    end
  end

  def self.default_description=(new_description)
    @@default_description = new_description
  end

  def self.default_description
    return @@default_description
  end
end