Behavior Driven Development

I was looking at RSpec(http://rspec.info/) as I want to use it in a open source project. The definition of RSpec says that its a Behavior Driven Development framework for Ruby. As a TDD practitioner, I have been writing tests using xUnit frameworks. This will be my first experience with BDD frameworks. But how BDD differs from TDD ?

In test driven development we write tests before we code. However, testing the code and application is not the sole purpose of writing tests.TDD is not only about writing tests. Its about writing specifications for individual piece of code in form of tests, which should (ideally) lead to better design. So TDD is about specifying requirements and design and not only testing. BDD makes this fact explicit. It uses proper vocabulary and has emphasis on using the proper words (e.g. specification instead of tests). BDD gives more emphasis on coming up with the behaviour and specification rather than tests.

BDD is not a radically new way of thinking (like TDD is). Its just a improvement, a next step in TDD thinking. It combines the aspects of Domain Driven Design with TDD. It defers from TDD in a sense that we specify behaviour of the application rather than testing its implementation.However, just like TDD, we should do it in small steps, writing down the specification for small part of app before the part is implemented.

BDD uses Ubiquitous language - language which is equally understood by developers and users. Rspec, which a Behavior Driven Development framework for Ruby, allows us to define specifications in language such as "should add two numbers" or "should have score greater than 50". This kind of specification is more intuitive than unit tests.

So in short, BDD frameworks allows us to write executable specification and acceptance criteria. However, I still have some questions like - Can we do BDD with xUnit frameworks which are defined for TDD ? Do we still need unit tests to test the application at granular level or we should use BDD framework for that purpose? Hopefully, answers to these questions will be clear once I become more familiar to BDD style.