The originial Diet Coke is sweetened with Aspartame (nutrasweet). Coke Zero is sweetened with a blend of aspartame and acesulfame potassium. Coca-Cola's position is that the Coke Zero is supposed to taste more like the original Coke, but without calories or sugar.

Marketing

Apparently, coca cola tried to combine diet coke and something else, to create coke zero. i agree, the labels are the same. try this for the real taste of coca cola like it use to be: 50% diet coke and 50% regular coke, both in the same cup. let me know how you like it!!

Additional info on the differences, from Wikipedia :

This product is different from Diet Coke because of the sweeteners it uses (Diet Coke only uses aspartame), and because, unlike Diet Coke, this product is based on the original Coca-Cola formula. Diet Coke was an entirely new formula which would eventually become the basis for the controversial New Coke.

Also, Coca-Cola Zero is not a new product. The same blend of aspartame and acesulfame potassium along with the Coca-Cola Classic formula has been available for quite some time in many European countries and is commonly called Coca-Cola Light.

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.

Nothing - i cant taste any difference! It just makes money!

Coke Zero doesn't seem as sharp though it does seem to have a bit more of a sodium/salty taste. I enjoy coke zero at about 48 degrees and diet coke at about 41 degrees. I am waiting for coke lean with about 80 calories per serving and no artificial sweeteners.

Many won't be seen on a diet,
as Diet Coke implies,
So by labelling it Coke Zero,
They get a profit rise

Coke Zero is sweetened with aspartame and acesulfame potassium (ace-k) and has zero calories. The only chemical difference between Coke Zero and Diet Coke is that Coke Zero has about half the aspartame but has more ace-k

The first answer is correct. I switch from time to time between them both. My hubby likes zero better-I prefer diet. Zero is closer tasting to original coke, however, close your eyes and drink between the two and you can definately tell the difference. :)