TDD is an inside-out development technique whereas BDD is an outside-in development technique. TDD mainly focuses on unit testing and integration test case automation.
BDD focuses on end-to-end functional test cases and user acceptance test cases. However, BDD could also be used for unit testing, smoke testing, and, literally, every type of testing.
BDD is an extension of the TDD approach; hence, BDD also strongly encourages test-first development. It is quite natural to use both BDD and TDD in the same product; hence, BDD isn't a replacement for TDD. BDD can be thought of as a high-level design document, while TDD is the low-level design document.