"Unit testing is a test that validates that individual units of source code are working properly", that's what Wikipedia says about unit testing. That's general knowledge.

But what motivates me even more than the increased software quality is that it saves me development time. This sounds odd as you might believe that TDD means writing more code.

A real-life example is included (an internal invoicing system) with the steps they'd follow to test it normally via the browser (slow) or through the automated tests (fast!)