The goal of testing in PowerShell is to ensure that the code works as intended. Automatic testing ensures that this continues to be the case as code is changed over time.
Testing often begins before code is ready to execute. PSScriptAnalyzer can look at code and provide advice on best practices. This is known as static analysis.
Unit tests pick up when the code is ready to execute. Tests may exist before the code when you are following practices such as Test-Driven Development (TDD). A unit test focuses on the smallest parts of a script, function, module, or class. A unit test strives to validate the inner workings of a unit of code, ensuring that conditions evaluate correctly, that it terminates or returns where it should, and so on.
Testing might extend into systems and acceptance testing, although this often requires a test environment to act against. Acceptance testing...