Mocks, Stubs and Fakes: it's a continuum
Highly recommended reading: Fowler's article Mocks aren't Stubs.
Generically called (by Gerard Meszaros's xUnit patterns book) Test Doubles, all three kinds of test aids are intended to replace real implementations of dependencies of the object under test. From Fowler's article, we learn the difference between them as explained by Meszaros: