Test-Driven Development, or TDD, is a software development process that favors testing a specific unit before developing the code. By testing first, developers can refactor and optimize the resulting code to produce the best possible solution.
The aim of TDD is to have developers create functionality that directly solves a specific test case. Failed tests are actually expected, because TDD demands that developers rewrite (or refactor) code until the tests pass in the most lean and lightweight manner.
TDD is beneficial for the overall software development process because it ensures that specific features are viable before the final build of the application is completed. By solving for specific use cases, developers can ensure that the final application will be able to handle specific end user requirements.
Because failure and refactoring is built into TDD, the process ensures that final code is as efficient as possible.
Because TDD is highly focused on specific features, it is not always a good solution for macro-scale features in an application, such as user interfaces or database operations.
Any testing which requires the full functionality of the software will not be possible with TDD, because each feature is built in isolation — that’s why it takes so long to do, too!
Lastly, TDD should not be confused with User Acceptance Testing. Units that are successful in TDD still need to be tested with end users once the code is written.