I guess the closest description would be it's similar to a lightly carbonated YooHoo. But better. Much better. It's more refreshing. You could make your own at home, I think the recipe is a squirt or 2 of chocolate syrup (U-Bet brand if you are a purist) and an inch of milk. Then fill the cup with carbonated water, stir and enjoy.
But if you don't want to fuss with all that, I highly recommend Jeff's Chocolate Soda Egg Cream.

Old stock photo
A 12 oz. bottle has 170 calories of mostly sugar. Personally, I'd rather make my own, or get one from the soda fountain, but if you have never had a genuine NY Egg Cream, Jeff's will give you an idea of how the final product should taste. While I never tried it, I'm sure about an inch or so of chocolate milk, mixed with seltzer would give the same results.