First, a little background
It's possible that you've had some exposure to unit tests in your career. It's highly likely that you've written a test or two. Many developers, unfortunately, haven't had the opportunity to experience the joys of Test-Driven Development.
John's story on TDD
I was first introduced to TDD about five years ago. I was interviewing for a lead developer position for a small startup. During the interview process, the CTO mentioned that the development team was practicing TDD. I informed him that I didn't have any practical TDD experience, but that I was sure I could adapt.
In all honesty, I was a bit nervous. Up to that point, I had never even written a single unit test! What had I gotten myself into? An offer was extended and I accepted. Once I joined the small company I was told that, while TDD was the goal, they weren't quite there yet. Phew; crisis averted. However, I was still intrigued. It wasn't until a few months later that the team delved into the world of TDD...
