The main difference is that the French version is baked inside a tart crust while the Spanish version is a pudding-like baked custard, usually baked in a water bath. The ingredients are pretty close, but the Spanish version usually does not have Cornstarch in it and is made with Milk only. The French have a similar version of the Spanish Flan called "Crème Caramel" (sometimes also called just "Flan").