What is mocking in testing and when should you use it?

Mocking creates fake objects that simulate real dependencies for isolated testing. Types: Mock (verifies interactions), Stub (returns predefined responses), Spy (wraps real object, records calls), Fake (simplified working implementation). Use mocking for: external services (APIs, databases), slow operations, non-deterministic behavior. Avoid: over-mocking leading to brittle tests, mocking what you don't own (use adapters). Frameworks: Mockito (Java), pytest-mock, Jest mocks. Balance isolation with integration testing.