A flan is a form of custard in Spanish cuisine and related styles, such as Cuban cuisine. It is characterized by being topped with a thin, flowing caramel sauce, but not having the caramel glazed to it, as with the French creme caramel. Often, it is served in a bowl, with enough sauce surrounding it to be spooned up after the custard, alas, is gone.
As with any basic dish, there can be a multitude of variations, to the basic pudding or the sauce. Several variants add rum or brandy to the sauce, or flavorings such as vanilla.