Output is a direct measure of total quantity produced, whereas yield is a measure of quantity of usable or quality end product based against total output. I hope this helps.

Ex: The factories production manager John was very proud of his department's production output. However, John's enthusiasm was short lived, when the next day, his quality control manager came to him and said "John, your department's production output is very good, but after analysis, many units have defects, so our yield is only 50% of our output, we must find out why!"