Test-Driven Development (TDD) is a practice for efficiently evolving useful code. While its name implies that it's mostly about testing, test-driven development is primarily about design: it keeps programmers focused on exactly what they need to build and helps them avoid over-engineering.
There are two levels of TDD:
Acceptance TDD (ATDD): With ATDD you write a single acceptance test or behavioral specification depending on your preferred terminology and then just enough production functionality/code to fulfill that test. The goal of ATDD is to specify detailed, executable requirements for your solution on a just in time (JIT) basis. ATDD is also called Behavior Driven Development (BDD).
Developer TDD: With developer TDD you write a single developer test, sometimes inaccurately referred to as a unit test, and then just enough production code to fulfill that test. The goal of developer TDD is to specify a detailed, executable design for your solution on a JIT basis. Developer TDD is also known as TDD.