Unit Tests' value in TDD

Hierarchy of tests:

• Acceptance: Does the whole system work?
• Integration: Does our code work against code we can't change?
• Unit: Do our objects do the right thing, are they convenient to work with?

Unit Tests' value in TDD:

• Write loosely coupled components
• Detect errors while the context is fresh in our mind
• Add an executable description of what the code does (documentation)
• Focus: discouraging unnecessary features
• Rythm: tests tell what to do next
• Trust: tests give a reason to trust you

ref:
- Extreme Programming Explained
- Growing Object-Oriented Software, Guided by Tests