A lot has been discussed about QC and QA but the way I explain to people is to use a story I read in my school days. The story is
One of the big enterprises wanted to replace the bulbs in their factories with a more efficient one which will save them a lot of power. They hear about a new company which specialises in those type of bulbs. The big enterprise places an order of 100,000 bulbs, the QC department adds a note saying that per 1000 bulbs they are willing to tolerate poor quality in about 6 bulbs which can be defective or broken but nothing more than this.
After few days they received 100,000 bulbs and another small package of 600 bulbs. The delivery note says “We have delivered 100,000 bulbs to you as per your order, we did not understand your QC requirements, so we have broken another 600 bulbs and attached with the delivery so that you are not disappointed”.
The bulb company had strong Quality Assurance while the expectation was just Quality Control. Both won’t understand each other because the goals are different. Under QC, you are trying to control how many defective items can go out in the production, always an after thought while QA is about how to ensure that output at every stage of development is of very high quality. QC is about find and quarantine, while QA is about prevent or treat.
If we want another analogy,
QC is about allowing only fit individuals to join sports, while QA is to train every individual to be able to join sports.