What is the difference between include and extend model in Ruby?

原文地址：Just getting my head around Ruby metaprogramming... the mixin/modules always manage to confuse me.

• include : mixes in specified module methods as instance methods in the target class
• extend : mixes in specified module methods as class methods in the target class

So is the major difference just this or is a bigger dragon lurking? e.g.

module ReusableModule
  def module_method
    puts "Module Method: Hi there!"
  end
end

class ClassThatIncludes
  include ReusableModule
end
class ClassThatExtends
  extend ReusableModule
end

puts "Include"
ClassThatIncludes.new.module_method       # "Module Method: Hi there!"
puts "Extend"
ClassThatExtends.module_method            # "Module Method: Hi there!"