What is the difference between unit testing and integration testing?

Unit testing and integration testing are both essential phases of software testing, each serving a distinct purpose in the development process.

Unit testing specifically targets individual components or functions of the software, isolating them to ensure they perform as expected. This testing is typically conducted by developers during the coding phase, focusing on validating the logic and behavior of single units in isolation. By confirming that these pieces operate correctly on their own, unit testing lays a foundational level of confidence in the software's building blocks.

On the other hand, integration testing takes this a step further by examining how these individual units work together as a combined system. It is designed to identify issues that may arise from the interaction between different components, ensuring that they communicate correctly and cooperate to deliver the intended functionality. This testing phase assesses the integration points and interfaces among modules, revealing potential problems such as data flow inconsistencies or interface mismatches that would not be apparent when testing units independently.

Understanding this distinction is crucial because both testing types address different layers of application reliability. Unit testing ensures each part functions correctly on its own, while integration testing addresses the overall coherence of the application when combined.