Mocking vs. Spying in mocking frameworks

Question

In mocking frameworks, you can mock an object or spy on it. What\'s the difference between the two and when would/should I use one over the other?

Answer 1

Spies have two definitions. One, is where the real method is called, another where, no functionality is called and only null or null equivalent values are returned, but methods were called, and they're state was recorded, commonly like, method x was called y times.