State Testing vs Interaction Testing
Highly recommended reading: Fowler's article Mocks aren't Stubs. It's a very clear characterization of the types of so-called Test Doubles (mocks, fakes, stubs, etc.) you can use to aid your unit testing needs, but also of the kinds of TDD you can do: classic (or state) TDD and mockist (or interaction) TDD.
I got the feeling that the article seemed a bit biased towards mockist TDD, only to see in the "So should I be a classicist or a mockist?" section that Fowler himself is a classic TDDer :o). Maybe it's because mockist TDD is a newer approach to TDD and therefore required a more extensive explanation. Or maybe it's just that the article title is a bit misleading as its content is about mocks in the strict sense used by mockists, and not in the general sense I think of them (as generic aids in unit testing)....