The terms “quality assurance” and “quality control” are often used interchangeably to refer to ways of ensuring the quality of a service or product. The terms, however, have different meanings.
Control: An evaluation to indicate needed corrective responses; the act of guiding a process in which variability is attributable to a constant system of chance causes.
Quality Control: The observation techniques and activities used to fulfill requirements for quality.
Assurance: The act of giving confidence, the state of being certain or the act of making certain.
Quality Assurance: The planned and systematic activities implemented in a quality system so that quality requirements for a product or service will be fulfilled.
At its simplest, quality control is inspecting, testing or checking something (service or product) to make sure it's OK. The intent is to identify anything that isn't OK, and either fix it or eliminate it, to make sure it conforms to the specifications, and has/does/functions as required. Quality control is typically done at the end of the line, before it 'goes out the door'. If the something isn't OK, this is called 'nonconformity' or a nonconforming service/product.
Quality control does not ensure quality, it only finds instances where quality is missing. Obviously it's better than nothing, but it has its limitations. The most important of these is that you only find out that things aren't OK at the end of the process.
Quality assurance developed from the realization that quality could be improved by looking 'further up the line'. Quality assurance activities are determined before production work begins and these activities are performed while the product is being developed. It is aimed at preventing nonconformities/defects.
Quality assurance focuses on processes and their continuous improvement. Its goal is to reduce variance in processes in order to predict the quality of an output (final or interim product), gather best practices for the company, reduce cost, and reduce time to market. Quality assurance is strongly linked to innovation and creativity. Quality Assurance neither imposes nor defines processes for other people, but it provides advice and support to the process owner, which leads to the ability to measure success and make decisions based on facts.
Quality control is concerned with examining the product or service — the end result and quality assurance is concerned with examining the process that leads to the end result. A company should use quality assurance to ensure that a product is manufactured in the right way, thereby reducing or eliminating potential problems with the quality of the final product.