Include vs Extend

include_vs_extend.rb

module Tweet
    def save
      puts 'Tweet saved'
    end
end

class Foo
  include Tweet # Add methods from Tweet as Instances Methods
end

class Bar
  extend Tweet # Add methods from Tweet as Class Methods
end

Foo.save # => undefined method `save' for Foo:Class (NoMethodError)
Foo.new.save # => Tweet saved

Bar.save # => Tweet saved
Bar.new.save # => undefined method `save' for #<Bar:0x007f813104e0e8> (NoMethodError)