During development of an application (web/software/mobile) quality assurance is any systematic process of checking to see whether a product or service being developed is meeting specified requirements.
Many companies have a separate department devoted to quality assurance. A quality assurance system is said to increase customer confidence and a company’s credibility, to improve work processes and efficiency, and to enable a company to better compete with others.
Quality assurance was initially introduced in World War II when munitions were inspected and tested for defects after they were made. Today’s quality assurance systems emphasize catching defects before they get into the final product.
Source – http://searchsoftwarequality.techtarget.com/