Flan is commonly used as a term to describe the Spanish or Mexican version of Crème caramel. However, traditional flan is both more rich and more dense than Crème caramel, as it contains more eggs and yolks as well as some milk, light whipping cream, half and half, and/or sweetened condensed milk. Historically, Flan was baked on top of a stove in a water bath. Now, it has become customary to cook the flan in a waterbath inside of the oven where it can bake "low and slow".

Spanish and Mexican Sweet Custard
Flan is an oven-baked caramel custard dessert that is a very popular dessert in Spain and in Mexico. It  is made with a top layer of custard paired with the sweetness of a light caramel sauce, that is put in the bottom of the pan underneath it. Both are baked together. When chilled and then inverted to unmold, the sauce pours over the custard and is served as is. The typical favoring is simply vanilla but there are numerous variations that include almonds, pistachio, orange, pumpkin, coffee, lemon, and various other fruits.

Flan may be prepared in a soufflé dish or in individual ramekins or flan dishes. In Mexico it is commonly served in 5 or 6 ounce ramekins. It begins with golden caramel that's made by boiling sugar and optionally, water to just the right syrupy consistency and pouring it into the bottom of a soufflé dish or individual ramekin. 

To make: stir sugar and water over low heat until sugar dissolves. Increase heat to high and cook WITHOUT stirring until syrup turns deep amber, brushing down sides of pan with wet pastry brush and swirling pan occasionally, about 10 minutes. 

Remove immediately from the heat when done and and pour it evenly over the bottom of the dish or dishes in which you will be baking the flan. Tilt the pan so that the caramel coats the bottom and a bit of the sides. 

A custard of eggs and milk (often flavored with vanilla) is then poured on top of the caramel and baked until it sets. When you take your flan from the refrigerator, you need to briefly warm up the caramel (it should still be fluid) so it will release from the pan. Dip dish in a baking pan of hot water 3 seconds. Run a thin knife around edge of dish and invert flan onto plate. Shake gently to release flan. Carefully lift off ramekin allowing caramel syrup to run over flan. 

Savory Flan, Another Variation
A small savory version of Flan can be found on many restaurant menus as an accompaniment to a main course. Examples are Sweet Corn Flan, Sweet Potato Flan, etc. These are typically made of eggs, cream, and the appropriate vegetable flavoring.

Classic Flan Tart
The Flan pastry is baked in a Flan ring (see picture) atop a baking sheet.  Flan may also be baked in a tart pan or a pan with a removable bottom.  A filling is added to the baked pastry. Fillings may be of any type but typically they are custard with a fruit topping or cheese custard resembling a Quiche.