Testing is necessary for software systems to ensure reliability, manage costs, and reduce risks. It is impossible to exhaustively test a system, so testing aims to detect defects and measure quality. Testing alone cannot improve quality but can identify issues to address. Different testing types exist for various stages, including unit, integration, system, and acceptance testing, and both black-box and white-box techniques are used. Rigorous planning, design, execution and tracking of test cases and results is needed. While testing shows defects, debugging is then needed to identify and address the root causes.