There’s less and less every day it seems. However there are two, maybe three base differences that more or less hold true. First, that gelato is made (or was once exclusively made) with milk. This is supposed to differentiate it from a French-style custard ice cream in that French-style is made with — no surprise — cream. However that general rule has come to be very loosely interpreted, often just “some” milk is enough to place a frozen dessert in the gelato category, cream making up the rest.
The other big thing people say about gelato is that it doesn’t have any air in it. Nonsense. Gelato has lots of air or “overrun” as they call it in the trade, up to about 25%. Granted this is frequently less than conventional ice creams, which can be as much as 50% air, though most ice creams-in-a-tub clock in at around 35%. Part of the difference in air content has to do with the way gelato is churned, which gives it more density and supposedly a richer and more flavorful texture.
And I suppose if you’re talking about real Italian gelato, much of that is made with milk that is either unpasteurized or has a bacteria culture added to it for zing. But these are all relatively minor points in my book. To me the thing that gives the impression of a dramatic difference between gelato and ice cream is simply that gelato is served at a slightly higher temperature. This makes gelato appear softer, creamier and more flavorful than conventional ice cream even though when it comes right down to it, it really isn’t very different at all.