Can you explain the differences between testing individual components and testing their interaction in the system?

Unit testing focuses on verifying the operation of small, isolated parts of a program — for example, individual methods or classes. The goal is to ensure that each component works correctly on its own. To do this, mocks or stubs are often used to isolate the tested component from the rest of the system.

Integration testing checks how different parts of the system work together. It is important here to ensure that components correctly exchange data and interact as expected. For example, testing the interaction between a service and a database or between two modules.

Thus, the main difference lies in the level of isolation and the goal: unit tests verify individual parts, while integration tests verify their joint operation.