Test-driven development (TDD) is a Computer programming technique that involves writing tests first and then implementing the code to make them pass. The goal of test-driven development is to achieve rapid feedback and implements the "illustrate the main line" approach to constructing a program. This technique is heavily emphasized in Extreme Programming.
Unit Testing in SharePoint has never been a simple task. There are numerous dependencies on the platform. Because of this, TDD has even more difficult. Practices of isolating the test from the SharePoint platform become complicated and can best be accomplished with the use of Mock objects.
External References
- SharePoint and TDD - The other view by Andrew Woodward

- SPTDD: SharePoint and Test Driven Development, Part One by Eric Shupps

- SPTDD: SharePoint and Test Driven Development, Part Two by Eric Shupps

- Jan Tore Lamoy published a guide for TDD in SharePoint in a 2 part series Part 1
Part 2
- Test Driven Development Tutorial
- On Typemock's site
