What is the difference between 'extend' and 'include' in ruby?

Answer 1

" ... include is the most common way of importing external code into a class ...

Include

include is the most used and the simplest way of importing module code. When calling it in a class definition, Ruby will insert the module into the ancestors chain of the class, just after its superclass. 
...

Extend

On the other end, using extend on a class will actually import the module methods as class methods.
... "

Source : ( code examples included showing implementation ) https://medium.com/@leo_hetsch/ruby-modules-include-vs-prepend-vs-extend-f09837a5b073

Answer 2

Extending is using a preexisting class or methods library and tapping into its resources to perform futher functionality. Include is importing a library or class type INTO the programming. Its same with Java and C++ except they use #include in C++ when its import in Java. The terminology may be slightly diff but concept and actions performed are same.

Answer 3

In Ruby, include mixes a module’s methods as instance methods in a class, whereas extend mixes them as class methods on the class or specific object.
https://www.railscarma.com/blog/ruby-include-vs-extend-understanding-the-key-differences/