Key differences between QA, QC and testing

Quality Assurance (QA) - A set of activities designed to ensure that the development and/or maintenance process is adequate to ensure a system will meet its objectives.

Quality Control: A set of activities designed to evaluate a developed work product. These activities can always include the audits conducted by the QA team to assess the cost of correcting defects, documentation etc.

Testing: Finding defects by executing a system/software. (Note that the "process of executing a system" includes test planning prior to the execution of the test cases.)