According to AgileAlliance.org:
BDD (Behavior Driven Development) is a synthesis and refinement of practices stemming from TDD (Test Driven Development) and ATDD (Acceptance Test Driven Development).
That’s great, but what does that really mean?
BDD has its roots in TDD, but supersedes TDD in the agile development process. BDD recognizes that User Stories are the framework for the development process and that these stories determine the necessary tasks required to satisfy given business requirements. Read more