TDD is not about unit tests
-- Dave Farley & Arjan Molenaar
On many occasions when we come at a customer, we're told the development team is doing TDD. Often, though, a team is writing unit tests, but it's not doing TDD.
This is an important distinction. Unit tests are useful things. Unit testing though says nothing about how to create useful tests that can live alongside your code. On the other hand TDD is an essential practice for improving the design of your code. These are very different things.