By the way, I am using TDD in my .NET projects, if you want to look at some example code, maybe this explains it better than ten paragraphs on the internals of NUnit:

My StringHelper class: StringHelper.cs
The unit tests for said class: StringHelper.Test.cs

For a to-the-point and very encouragingly written explanation of TDD and more agile development concepts, this book is worth its weight in gold

Agile Principles, Patterns, and Practices in C#