What is unit testing and why is it important?

Unit testing verifies individual components (functions, methods, classes) work correctly in isolation. Tests are automated, fast, and repeatable. Importance: catches bugs early (cheaper to fix), documents expected behavior, enables safe refactoring, supports continuous integration. Good unit tests are: fast, isolated, repeatable, self-validating, timely (written with code). Frameworks: JUnit (Java), pytest (Python), Jest (JavaScript). Aim for high coverage of critical code paths.