I ran across a nice article by Mark Seemann on MSDN. It was published in the September 2007 issue of MSDN Magazine. In it he covers what he describes as the continuum relationship of Mocks, Dummies, Stubs, Spies and Fakes in Unit Testing.

The following diagram is from his article to illustrate the point:

As you can see, each component has varying levels of implementation depending mostly on your needs etc. The article is worth a read!

Cheers,

Robert Porter