The difference is both the artificial sweetener and the syrup formula. Diet Coke is sweetened with Apartame. Diet Coke's syrup formula is slightly different from Coke Classic. The Diet Coke formula was introduced with a high fructose sweetener instead of Aspartame and labeled New Coke two years after Diet Coke was introduced. New Coke was soon abandoned, but Diet Coke is still produced. Coke Zero is sweetened with Aspartame and Acesulfame Potassium. It's syrup formula is the same as Coke Classic's. Coke Zero is supposed to be a no calorie equivalent to Coke Classic and avoid the the word Diet in its name. Diet Coke is a no sugar equivalent to the now discontinued New Coke product. The difference in taste comes from the differences in the syrup formula and the artificial sweetener.