include_vs_extend_in_ruby.rb

module Foo
  def bar
    puts 'foobar'
  end
end

class IncludeModule
  include Foo
end

class ExtendModule
  extend Foo
end

class Plain; end

IncludeModule.bar # => undefined method `bar' for IncludeModule:Class (NoMethodError)
IncludeModule.new.bar # => 'foobar'

ExtendModule.bar # => 'foobar'
ExtendModule.new.bar # => undefined method `bar' for #<ExtendModule:0x007f848197aee8> (NoMethodError)

plain = Plain.new
plain.bar # => undefined method `bar' for #<Plain:0x007fb252086f90> (NoMethodError)
plain.extend(Foo)
plain.bar # => 'foobar'

new_plain = Plain.new
new_plain.bar # => undefined method `bar' for #<Plain:0x007fb252086f90> (NoMethodError)