Testing involves double checking software to find defects. There are many types of tests, including unit tests to test individual code modules, integration tests to test system components working together, and validation/verification tests to ensure requirements are met. Tests should check for resource exhaustion, errors, performance under different loads, and usability. Profiling and usability testing tools can help analyze software performance and user experience. Effective usability testing involves representative users and both quantitative and qualitative feedback.