Unit testing is a part of Test-Driven Development, but TDD is not only about Unit testing. In TDD, you usually write tests before you write any application code, and these test serve as documentation. However, Behavior-Driven development is a less well-defined term. Some contrast BDD and TDD while others say that they're they are like two sides of the same coin.
Bob Silverberg is the principal consultant at SilverWare Consulting. In this interview he discusses Test-Driven Development and Unit testing. Silverberg says that the two techniques sometimes get confused with one another.