What is the difference between unit testing and integration testing?

Unit testing verifies one function, class, or module in isolation, often with mocks for databases, APIs, and external services. Failures point to a small code unit, so feedback is fast and debugging is straightforward. Integration testing checks that two or more real components work together - service plus database, API plus queue, frontend plus BFF - without mocking every boundary. It finds contract mismatches, serialization bugs, and configuration errors units cannot see. Units are numerous and cheap to run; integrations are fewer and slower but closer to production behavior. Healthy teams rely on both: units for logic and regressions at the code level, integrations for wiring and data paths.