I think the most important difference between them is their intentions.
Let me try to explain it in WHY stub vs. WHY mock
Suppose I'm writing test code for my mac twitter client's public timeline controller
Here is test sample code
twitter_api.stub(:public_timeline).and_return(public_timeline_array)client_ui.should_receive(:insert_timeline_above).with(public_timeline_array)controller.refresh_public_timeline- STUB: The network connection to twitter API is very slow, which make my test slow. I know it will return timelines, so I made a stub simulating HTTP twitter API, so that my test will run it very fast, and I can running the test even I'm offline.
- MOCK: I haven't written any of my UI methods yet, and I'm not sure what methods I need to write for my ui object. I hope to know how my controller will collaborate with my ui object by writing the test code.
By writing mock, you discover the objects collaboration relationship by verifying the expectation are met, while stub only simulate the object's behavior.
I suggest to read this article if you're trying to know more about mocks: http://jmock.org/oopsla2004.pdf