What’s the difference between unit, integration, and end-to-end (E2E) tests?

**Unit tests** validate small units (functions/classes) in isolation. **Integration tests** validate interactions between components (e.g., service + DB). **E2E tests** validate full user flows through the system (UI → API → DB). **Best practice:** follow the testing pyramid—many fast unit tests, fewer integration tests, and a small number of E2E tests for critical paths.