gistfile1.rb

class Bar
  def self.bar
    @bar ||= "bar"
  end

  def self.baz
    @@baz ||= "baz"
  end

  def bar
    @bar
  end

  def baz
    @@baz
  end
end

Bar.bar     # => "bar"
Bar.new.bar # => nil
Bar.baz     # => "baz"
Bar.new.baz # => "baz"