O'Reilly logo
  • David Francis thinks this is interesting:

There is a difference between include and extend in this Order object model: include puts the module’s methods on the calling class as instance methods, and extend makes them into class methods.

From

Cover of Rails™ AntiPatterns: Best Practice Ruby on Rails™ Refactoring

Note

http://culttt.com/2015/06/10/understanding-class-methods-verses-instance-methods-in-ruby/