Integration tests in Magento test functionality across components and services by interacting with real classes and dependencies, unlike unit tests which isolate classes through heavy use of mocks. They can test full workflows like customer registration or validate configuration. While slower, integration tests are more reliable and require less maintenance than unit tests. The ReachDigital test framework improves integration test performance by disabling reinitialization and config loading. It also allows mocking 3rd party services through test module preferences.