Test levels refer to the different stages or layers of testing that are performed sequentially or in parallel to ensure the software's quality. Examples of test levels include unit testing, integration testing, system testing, and acceptance testing. Each level focuses on specific objectives, such as verifying the functionality of individual components, testing the interaction between components, evaluating the overall system behaviour, or validating the software against user requirements.
On the other hand, test types refer to the different categories or dimensions of testing that address various aspects of the software's quality. Test types are not strictly tied to a specific test level. They can be applicable at multiple levels or even span across the entire testing process. Examples of test types include functional testing, non-functional testing (e.g., performance, security, usability), regression testing, and user acceptance testing.
While there may be some overlap between specific test levels and test types (e.g., system testing often includes functional and non-functional testing), they are not hierarchical in nature. Both test levels and test types need to be considered and incorporated into a test strategy to ensure comprehensive coverage and achieve the desired quality objectives.