mixins.rb

module SomeMixin
  def doit
    puts "HELLO!"
  end
end

class IncludesMixin
  include SomeMixin
end

class ExtendsMixin
  extend SomeMixin
end

mixins2.sh

2.0.0-p247 :016 > IncludesMixin.doit
NoMethodError: undefined method `doit' for IncludesMixin:Class
	from (irb):16
	from /Users/davec/.rvm/rubies/ruby-2.0.0-p247/bin/irb:16:in `<main>'
2.0.0-p247 :017 > IncludesMixin.new.doit
HELLO!
 => nil 
2.0.0-p247 :018 > ExtendsMixin.doit
HELLO!
 => nil 
2.0.0-p247 :019 > ExtendsMixin.new.doit
NoMethodError: undefined method `doit' for #<ExtendsMixin:0x007ffdbb22d1c8>
	from (irb):19
	from /Users/davec/.rvm/rubies/ruby-2.0.0-p247/bin/irb:16:in `<main>'