Module (or unit) — this is part of the functionality of the application, the result of which we can check (or test).
Unit Testing — verifying that the module works exactly as expected. Having written tests once, whenever you make changes to the code, you will only have to run the tests to verify that everything is correct. Thus, you will always be sure that with your changes you will not break the system.

TDD (test driven development) — The methodology used in software development.
First, tests are written. [konec_stroki] After the tests, you write application code that will pass these tests.