What is QA Testing?

Quality Assurance testing matches what is produced to what is expected. For example, a soda machine is supposed to fill cans to 16oz. It will never be exactly 16oz, but it will be close. Part of the QA process is developing clear boundary conditions beyond which a system is classified as having failed, and therefore is in need of tuning, redesign or other forms of maintenance attention.

Where products involve a user, QA testers explore the space of what can be done to/with the product, looking for things the designers overlooked, and places where the limits can be exceeded.

The job is basically a civilized version of getting paid to break stuff.