What is the difference PNG, PNG 8, 20, 50, 80, 100

Ok in working on a web page in Ai. i nocise there is a tab that will let you save you assets into a file so i been playing with this. But i dont know what are the difference in th PNG setting? what does the 8, 20, 50, 80, 100 Mean and whe is it best to use them?