Are you confused with Test driven development and Behaviour driven development?
The testdriven development is Based on the Unit testing of each individual module,
That may be a page rendering, may be model validation triggers, controller functionality, view level component finding etc.
Each and every minor tests are will get write before we starting the Functioality implementation.
When BDD and TDD are executing

  1. TDD runs before we start the coding (Functionality implementation)
  2. BDD Runs after the Functionality implementation.

How its helped in Agile

  1. Programmer and testers can able to contribute to the Testcase prepareation, based on the test case a programmer can able to code and implement the functioality.
  2. Its continues Testing process
  3. Less chance to