TDD

Test-driven development (TDD) is a software development process that relies on the repetition of a very short development cycle: first the developer writes a failing automated test case that defines a desired improvement or new function, then produces code to pass that test and finally refactors the new code to acceptable standards. Kent Beck, who is credited with having developed or 'rediscovered' the technique, stated in 2003 that TDD encourages simple designs and inspires confidence.

Test-driven development is related to the test-first programming concepts of extreme programming, begun in 1999, but more recently has created more general interest in its own right.

Programmers also apply the concept to improving and debugging legacy code developed with older techniques.

Is an evolutionary approach to development which combines test-first development where you write a test before you write code to fulfill that test and then refactoring the code. Test-driven development gives rapid feedback and allows to write cleaner code.

Citations: 
http://en.wikipedia.org/wiki/Test-driven_development