BDD means testing that you've delivered what the customer asked for. User stories expressed as test. This is like the architect's rendering of a building; it expresses and implies certain desired qualities.

TDD means testing that you've implemented a system which has all the significant qualities you expected. It intends to prove that the system does all of the right things and none of the wrong things on the way to delivering what the customer asked for. This is more like the blueprint of the building, making all significant details explicit.

You could practice BDD without TDD, though I've yet to see someone choose to. You can practice TDD without BDD, which I've seen many people do.