scopes.rb

$global_var = 'global'

class Foo
  def self.class_var # note name start with 'self.' making this a class-level method
    @@class_var
    # would be error if tried to access @instance_var here since class knows nothing about any particular instance of class
  end
  def instance_var
    @instance_var
  end
  def both_instance_and_class
    [@instance_var, @@class_var]
  end
  def initialize
    @instance_var = 'instance'
    @@class_var = 'class'
    local_var = 'local' # does not exist after this block of code is exited
  end
end

foo = Foo.new
foo.instance_var #=> "instance"
Foo.class_var #=> "class"
foo.both_instance_and_class #=> ["instance", "class"]
$global_var #=> "global"