Unit tests check individual units of software (be it functions, methods or classes) in isolation from the rest of the system. The goal is to ensure that every module works as intended.

1. Units tests ensure quick feedback.
2. They give confidence in refactoring.
3. They document code's behavior.
4. They can help find errors at the early stages.

• FIRST: Fast, Isolated, Repeatable, Self-validating, Timely;
• AAA: Arrange, Act, Assert;
• GWT: Given-When-Then.

Pros:

• quick feedback;
• behavior documentation;
• fast completion.

Cons:

• do not test module interaction;
• require support with changes to API;
• do not find integration errors.