Behaviour-Driven Development or BDD is a reformulation of Test-Driven Development expressed in terms of desired behaviours. It is not fundamentally different from existing TDD best practices, but the vocabulary employed is intended to gently guide people towards those best practices by making it easy to think about things the "right way".
An example of a BDD framework is RSpec. I’ve used it in the development of pretty much every Ruby project I’ve ever written (see Walrus and Wikitext for examples; the Rails application that powers this site, which isn’t yet open source, also heavily uses RSpec).
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