Quality Assurance and testing

The following explanations should be helpful in understanding the difference between Quality Assurance and Testing

Quality Assurance:

A set of activities designed to ensure that the software development process is adequate to produce high quality product. Quality assurance activities also involve monitoring and evaluating the process and thus ensuring continuous improvement.
It would be appropriate to say that Quality Assurance is a preventive mechanism.

Testing:

Software testing is a process comprising a set of activities performed with the intent of evaluating the software against its requirements under specified conditions.
Some times software testing is performed as an ad-hoc activity without having any systems in place. These kind of practices are not recommended because the results are not traceable, some times not repeatable. For this reason software testing will be considered as a process in this book not as a single activity.