The word
mock is generic. Often the words stub, fake, and double are used to mean the same thing. To be precise, we could make a distinction between mocks and stubs. A mock is a fake object that stands in the place of an actual object. A stub is a fake method that is executed in place of an actual method. In practice, the terms are used interchangeably. In fact, our Object#mock method should have been named Object#stub if we were being consistent.
RSpec uses the method double to handle mocking. This is aliased as stub and mock, which can be confusing, but these aliases are both deprecated, so you shouldn't use them. There is also a method called spy, which is a convenience method based on double that I encourage you to avoid. All of these are defined in rspec-mocks, RSpec's test double framework.
You can think of double as returning a dummy object. For very simple cases, we could simply use Object.new instead. For example, in our shopping cart test, we didn't do...