Pudding relies on starch, usually flour or cornstarch, to thicken milk to a creamy consistency. Custard has a similar creamy consistency but relies on eggs or egg yolks rather than starch to thicken it. Mousse has a lighter and airier texture than pudding and custard. Basically, if you add whipped cream to a custard base to lighten it up, it becomes a mousse. For this reason, this chocolate satin pie filling is considered a chocolate mousse.