TDD, BDD, DSL...

TDD - Test Driven Development is usual associated with Unit Tests.  

1. Write tests before creating any application code.
2. Write code
3. Run code with tests to verify it works
4. Repeat to add more functionality to your code

BDD - Behaviour-driven Development is an Agile development process that encourages collaboration between team members.   BDD combines TDD with ideas from domain-driven design (DDD) and object-oriented analysis and design to deliver software.   Get you requirements into User Stories, and develop Acceptance Criteria (I like Gherkin).  This ensure the "The Three Amigos" product owner/business, analysts, testers and programmers are on the same page (note these roles in Agile are often all performed by 1 person).

Domain Specific Language (DSL) - Language used to help communicate a systems behavior or share information such as User Stories and Gherkin,  DSL is very similar to a General Purpose Language such as Use Cases.