Include: Include it is adding methods of a class to an instance which we created.
Extend: Extend it is for adding class methods itself.
If we go with a simple example we can get a clear picture
puts ‘upgrade to new rails version’
Movie.new.advertisement # upgrade to new rails version
Movie.advertisement # NoMethodError: undefined method ‘advertisement’ for Movie:Class
Game.advertisement # upgrade to new rails version
Game.new.advertisement # NoMethodError: undefined method ‘advertisement’ for #<Game:0x1e708>
So that from above example we can get a basic idea that “include” makes the advertisement method available to an instance of a class
and where as the extend makes the advertisement method available to the class itself.