Actually, TDD is the way you figure out what you are building. People who start with a solution in mind may write tests first, doing a kind of pseudo-tdd, but they are not reaping the benefits of letting the requirements drive the design.

This is NOT TDD:

1 - Think of a solution

2 - Imagine a bunch of classes and functions that you just know you’ll need to implement (1)

3 - Write some tests that assert the existence of (2)

4 - Run all tests and fail

5 - Implement a bunch of stuff

6 - Run all tests and fail

7 - Debug

8 - Run the tests and succeed

9 - Write a TODO saying to go back and refactor some stuff later.

see: http://cumulative-hypotheses.org/2011/08/30/tdd-as-if-you-me...